Now this is an interesting book - I read it some time ago and browsed through it again last week, and found it still holds up. Yes, the ending is a little abrupt given how good the rest of the book is, but it's still a rare example of cleverly using alternate history to fit into an existing genre - in this case the 'heist and cast-and-mouse chase' crime thriller.
The alternate history is quite well done, with perhaps a few far-fetched or slightly eyebrow-raising butterflies employed; but given that Sealion is usually a monochrome 'Invasion Succeeds or Invasion Fails', the idea of a grudging armistice with the threat of an invasion if it fails is a nice take on the scenario. Especially when you consider that it, oddly, seems more 'realistic' a take than the usual idea that Sealion would just be an outright invasion.