How important is the Arts and Sciences course at UCL to you?
EPQ will pull time away from your studies, and you'll still have to do extra reading, so it seems like quite a lot on top of those. It would be good to do though, given the course you're applying to, because it's relevant and a nice way of combining both side. When do you actually do the EPQ? Deadlines and so on, because some people seem to get it over and done with reasonably quick if they just focus on it solidly for a small period of time. You have experience in writing essays and constructing arguments so I can't see it being overly challenging.
For Oxford, I don't think either way will matter too much, as you'll be doing the HAT, and they'll look at that more than whether you have an EPQ or not. Not every applicant is taking EPQ, nor are they all doing 4 a-levels, so they're both just a bonus.
I ask about importance, because i'd be tempted to drop the chemistry and continue with the EPQ, unless you're really very fond of chemistry. I imagine you're not so likely to firm the course, but was it an insurance thought?
Ultimately, I think another A-level takes more time than an EPQ. I kind of think that time would be better spent reading and exploring history and French, in terms of your application, and for the future. It means come exam time you can focus on the areas that really matter. It's not worth risking - as presumably you'll be needing AAA at minimum. The only other way i'd say it's useful is if you're more likely to get an A in it than English.
It's a personal choice. Depends how motivated and time efficient you are too.
Edit: seen above. If UCL is your goal then sod the EPQ and continue with four.