My daughter and several of my students have done undergrad dissertations over the past few years, so hopefully I can offer some guidance.

It'd be helpful to start with if we could know more about the parameters, but the ones I've been involved with at least observing and offering guidance on have tended to require a dissertation of about 12,000 words that's based chiefly on primary sources. The idea is that this is researched, typically, during a summer and written up over the course of a term that is supposed to be devoted to this purpose – you may be facing something different. But in this sort of case, typically the research portion of the work is supposed to occupy about 6-8 weeks at most and the secondary reading/writing process about another 8 weeks.

This is not as much time as it seems to be.

You ought to have had a significant amount of guidance before you even started the project from an academic whose main job it is to advise on choosing a practical amount of work, and a manageable topic. You seem not to have had that, but, essentially, the problem you are confronting is that you have chosen a topic that is more suitable to the sort of time available for a PhD thesis than an undergrad thesis, and even then would need be honed and focused quite a bit to make it fit. There's fundamentally no way any undergrad thesis can address a topic so gargantuan as "Thatcher's policy towards the EU, 1979-1991". You need to stop right now and consider a much more limited topic. I'd definitely want to start by consulting whatever undergrad handbook/guidance you may have been given about this part of your degree by your university.

I think, to begin with, consider exactly what type of dissertation you are interested in. You speak of wanting to write about policy, but you've started with a deep dive into budgets and figures. To even understand what those figures are telling you, you need to take a step back and start by looking at some policy discussions.

So my broad advice would be as follows:

1. Don't start by going straight to the documents. You have to be able to contextualise them and at the same time you need some guidance to what's available, so start with some targeted secondary reading. Read the relevant parts of Moore's recent bio of Thatcher and at least a couple of specific academic studies of Britain's relation with the EU in the period you are interested in. Pay special attention to the footnotes that tell you where the info in the books you're reading has been drawn from. You ideally want to start by identifying a couple of key policy documents, or Thatcher speeches, that set out what her ideas and her aims were, and be reasonably confident you understand how she got to that point ... what known problems, or issues, and what known advice or arguments, she'd engaged with in formulating the policy she tried to put into place. And you'll then need to follow through on how she went about implementing that policy, what difficulties she face, how all that influenced the way in which the policy was actually implemented, and what the differences between where she started, and where she finished, actually were, and why those changes occurred, and what sort of difference it made that they did.
2. Use all this to reduce your focus to a topic that is [a] manageable but also [b] is somewhere you can make something of an original contribution. Your suggestion of the budget rebate seems a reasonable one at first glance with respect to [a] but you also need to have something specific to say about it – something new. Ideally, you need to address a previously unconsidered aspect of the problem, or come up with a fresh interpretation that resolves some issue that has been discussed and disputed by other historians who have a broader focus.
3. Especially at the moment, with the problems of getting access to archives, make absolutely sure that the vast majority of what you need, if not everything you need, is going to be available to you digitally.
4. You are going about the thesis entirely wrong if you are in any way considering it as a narrative. It needs to be an argument, and it needs to be as focused on analysis, evaluation and interpretation as you can make it. It should have as little narrative to it as you can possibly manage.
5. Once you've completed a few days of secondary source reading, you ought to be in a better position to know what's most interesting about the broad topic you have chosen, where the gaps in our knowledge are, and what sort of focus you might be interested in. Then narrow from there to a topic that is manageable in the time you actually have available.

I'm sorry to hear all this is stressing you out so much and if it would help, we can talk a bit more about this on PM.