Really good info from M above already.
Apparently, the lobby of Red Bull Racing HQ in Milton Keynes is open to public, but do check before you go. Williams has a really nice museum with their old cars on display but I don't think you can just enter unless you have business to be there; maybe Captain can confirm. All other teams are off limits as far as I know. If you come to Brackley you will be able to take a picture of this nice sign outside the gates:
I just went to my first Sunday Scramble at Bicester Heritage earlier this month. I think the next ones are early January and late April. Apparently there was a whole section of car club parking which I missed, where people bring their normal, everyday drivers like 993RS and so on. Coming from Canada, many of the cars, even relatively mundane ones, were new to me, so it was pretty cool. You can show up in chill gear or you can dress up like you're going to Goodwood Revival, your choice.
I think BMW/Mini Park Lane usually has a few funky cars on display. There is also the British Motor Museum for a decidedly non-Porsche experience.