So ... you're living in a part of the world that has way more of 2 features I found very difficult when my kid was little (and I have just the one) -- short days (limited daylight) and cold and/or rainy weather. Two things I did to deal with times when those were noticeable was (a) join the indoor pool in our town and (b) go to big box home repair stores and let DS pretend he was riding the lawn tractors. Oh, and we have a really utterly fabulous children's museum with very good value annual memberships.
The internet being what it is, I can see that Oxford has a Storytelling museum and that there is a Bicester Leisure center with an indoor pool that looks like it has reasonably priced monthly passes -- though I have no idea whether it is kid friendly, whether you have a car, whether these are your kinds of "things." But really for me, getting out of the house was essential, and those things worked (and ... up to a point, a tired kid is a good kid...). And neither really presented much shopping appeal to me, or the kid.
Also, when I was a kid my mother used not to take any money at all when she'd take us to the pool, and then could tell us truthfully she had no money for snacks. I don't generally do that, but I don't hesitate to tell my kid, "I didn't bring any money for buying snacks," which is true, even when I do have money. I'm not advocating not buying (or providing) kids snacks, but it's also OK to say no to requests. And one piece of advice, once you have said no (in any given instance) do not change your mind, as that sets you up for being on the receiving end of badgering, ad infinitum.