Back then you were allowed to fly private/VFR/day on a foreign PPL, without formal ratification - you were almost certainly legal EXDAC. It has only got difficult for foreign licence holders the last few years.
Regarding airspace. Yes, in the USA airspace is accessible and the system make reasonable logical sense. British, and to a similar but perhaps lesser extent, European airspace is just an awful clutter of complexity, particularly with a lot of relatively low class A. Airways exist in much the same way as in the USA, but unlike the USA are not mostly in class E - they're mostly in class D (which you can't enter here without permission) and class A (which like everywhere else, requires an IR). On the other hand we also have the ability to fly IFR, potentially not talking to anybody, in class G, of which the UK has a great deal, particularly when you get away from the crowded SE of England. This will be one of the hardest things to get your head around when touring in the UK and Europe, but in reality - you just use the chart carefully and it'll all be fine.
It isn't true that all airways are class A in the UK, that is a misunderstanding by many PPLs, albeit an understandable one.
Regarding costs - rental on a typical club 4-seater in the UK is around £150-£200/hr. In a large syndicate, flying regularly, you can probably get your flying bill down to around £100/hr. Landing fees around £15, approach fees around £20, club membership at most clubs in the range £150-£400, depending upon where it is. Some flying clubs don't charge landing fees for resident aircraft, some do.
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