I think that having a roughly French DOM/TOM style arrangement is a bit awkward, for a couple of reasons. The first is that, other than Malta, I can't see that there all that many that would obviously be willing to stay in the UK, even under a more equitable and less colonial arrangement. Maybe if there's a big and obvious faceplant from a country declaring independence. With what Alex is suggesting plus Malta we've got maybe another 100,000, 200,000 at a stretch, in the 60s. Call it two Maltese constituencies, a Carribean constituency or three depending on who exactly is in (I'm guessing the big players, Jamaica and so on, are still out), maybe a Pacific/Indian ocean constituency. Lets be a bit "optimistic" and keep both Belize and Guyana as an analogy to French Guyana because why not. I think this takes us up to the best part of a million at indepence, and probably about 3 million now (I'm assuming greater immigration into the UK mainland in this case), which is of the same order as France's DOM/TOMs. We're already quite implausible here.
Then we run into a different problem: Hong Kong. I can't see a UK that hangs onto Malta, Belize, Guyana, maybe The Gambia, maybe Mauritius, a bunch of the Carribean, as one that is likely to let Hong Kong go. This UK is likely one that is more able to hang onto Hong Kong and if the former are integrated there would be fairly substantial calls to integrate the latter. Equally, a UK that gives Hong Kong up is likely not going to hang onto the rest of them and we'd be seeing renewed calls for independence at this point as a large chunk of the advantage of being part of the UK is gone - I actually think this is the most likely scenario. But let's say that either the UK is stronger, or China is weaker, or the UK believes sufficiently little in China's ability to take and hold Hong Kong and/or its ability to govern it in a "two systems" style. But we can't have China be too weak or else Hong Kong will be asking for independence rather than union.
Now in 1960 Hong Kong had a population of 3 million, or more than Northern Ireland. Now it's up at over 7 million, higher than any Home Nation besides England. At this point, we have enormously changed the nature of the UK and we've got enormous geopolitical butterflies, not least because it is likely something fairly nasty has gone down in China by this point one way or another (nastier than OTL I mean).
So basically I can see another 200,000 people being added, with maybe another 10 MPs worth, or else we've got a 5th Home Nation, with a 6th made up from the rest (ignore the Frankenstein of a nation that is Malta+Guyana+Belize+Mauritius+a bunch of other smaller islands). We're looking at a UK with a total population upwards of 80 million, where over 10% of MPs would represent "distant" constituencies, give or take a bit with devolved legislations which would obviously be needed. I think that this is an interesting idea but possibly not in the spirit of the OP?