The weight of population and resources in favour of a united England and against Wales would inevitably mean that the Welsh would be overwhelmed (in the long run rather than the short run if they had managed to be led by a good general or had overseas help) and be pushed back once England was united - ie after the union of Wessex and Mercia and reconquest of the 'Danelaw' / Northumbria in the 910s. The most serious attempt by York Viking rebels plus outside help from the Scots, Strathclyde (then a part Celtic, part Gaelic sub-state and ally of Scotland) and the Viking state of Dublin to overthrow the conquest of all N England by Wessex-Mercia was in 937, led by the capable and well-resourced Dublin warlord Olaf Guthfrithson, and once King Athelstan of 'England' (ie Wessex/ Mercia/ Danelaw), ruling from 924-5-939, had defeated this at the battle of Brunanburh the union of England was relatively secure and so the Welsh were at a permanent disadvantage - though York rebelled again with overseas Viking help in 939-44 and 947-54 and on the first occasion Olaf G helped them, took York and temporarily took the Danelaw too but soon died in battle. In 937, 941-44 and 954 the new 'English' state defeated the rebellion and retook York to restore the union by force , winning each time - presumably due to greater resources and operating on interior lines of communication as well as having better (royal) generals and a more coherent army.
After this the Welsh were at a permanent disadvantage and so likely to lose any insecure frontier territory (eg Flintshire, Herefordshire or lowland Gwent) in the long run even before the advent of cavalry and secure walled castles for England with the Norman invasion in 1066 tipped the balance further. Added to this, Wales was usually divided into four separate dynastic kingdoms - Gwynedd in the N, Powys in the centre (G and P were united from 844 onwards to the 1060s), Dyfed/ Deheubarth in the SW, and Glamorgan plus Gwent in the SE), and occasional unity by means of conquest and/or a dynastic alliance was rare and not long-lasting. The dynastic union of G, P and D in 942-50 under Hywel 'the Good' was by consent not conquest by a brilliant general, and pacific lawgiver Hywel kept out of the coalitions against the English both as king of Deheubarth in 937 and as king of all Wales after 942; in 1055-63 Wales was reunited by force by Gruffydd ap Llywelyn of Gwynedd, a highly capable general and politician, and he even forced King Edward 'the Confessor' to return Herefs West of the Wye to him by treaty but he was then overthrown by Harold Godwinson by invasion from England in 1063. Even an insecurely united Wales under a good leader - who was still resented by excluded rivals and at risk of a revolt at any point - could not prevail against England if E was united, as in the 940s and in 1063, so it was not just the Norman Conquest that tipped the scales.
But until the union of Wessex and Mercia under Alfred the Great's son Edward in the 910s there is a possibility of a stronger Wales holding onto wider territories - if it had been one state not four and at that one state run by a succession of strong single leaders with good armies and funds, , which would mean a different inheritance-system so that only the eldest son of a reigning king has the right of succession not all his sons, as in England. To do that you would need a more Romanised Wales, ie more towns and more Roman landlord dynasties plus a 'Roman' as well as tribal 'Celtic'/ British literary and legal culture, surviving from the C5th onwards , and so the predominance of a stable kingship looking back to the Roman Empire as its origin plus Roman law . Also, the small tribal-based kingdoms of post-Roman Wales would have had to be eclipsed before the chaos of the Viking raids in the C9th by one state under one dynasty,or at least to recognise one state as their national leader - the best chance for this lay with the strongest military power of the C6th, Gwynedd in the N, which had a 'national 'leader in Maelgwyn in the 520s-40s and later a powerful warlord, Cadwallon, and a capable army in the 620s-30s which was able to wipe out the Anglo-Saxon army of neighbouring Northumbria,kill its king Edwin, and ravage Yorkshire and the NE in a massive onslaught in 633-4. If the successes of Gwynedd in the 630s - soon reversed by a Northumbrian revival - had led to a long-term 'national' and better-resourced Welsh state in the C7th, possibly far wealthier and trading with the Mediterranean via SW Welsh ports surviving from Roman times, and then the Welsh had fought off the Vikings due to having a coherent fleet (big 'Ifs', all of this), then Wales might have been in a better position to hang onto a stronger frontier in the East. In the long term, the best chances for Wales would be a stronger surviving 'Roman' ethos and greater resources in the C5th and C6th fostering a coherent state - as united by greater long-term success for their post-Roman warlords fighting the new 'Anglo-Saxon' kingships emerging in lowland SE Britain, with or without any 'King Arthur'?
The failure of England to unite,or at least do so so quickly, would help a lot - eg if the early
kingdom of central England,Mercia, had been destroyed as a coherent force permanently by its Northumbrian foes in their clashes in 642 (Maserfelth,prob Oswestry) or 655 (Winwaed, near Leeds) and broken up - the Northumbrians did this in OTL 655 but it revived in 658 - so the Welsh did not have a major power on their border. No Offa, no Offa's Dyke - a R Severn frontier? Or Cadwallon finishes off
Northumbria (then still with a considerable Welsh/ Briton population in the Pennines and Lancs/ Cumbria)
and sets up a group of sub-states there as his vassals, incl Cumbria (indep British state to the 610s)and
Elmet and so secures Northern resources and allies to fight Mercia. The unwieldy new Northumbrian state of the early C7th nearly fell to Gwynedd, so if this is permanent then we have a 'Celtic'-led political order N of the Humber and the balance of power in the region is shifted long-term - provided that Gwynedd has the leadership to keep the Anglo-Saxons as his military inferiors and can use their mutual rivalries to keep them disunited.
If the Vikings destroy King Alfred in 871 or 878, Wessex stays broken up and England is a group of feuding Viking lordships, so a united Wales could stay as a regional power for longer. But in the long term, the advantages would still lie with England.