With regards to the question in the thread title - yep, there's a big difference.
I did Edexcel Religious Studies at GCSE, and then Edexcel's Philosophy&Ethics 'A'-Level course. I went to a Roman Catholic school, so the RS syllabus at GCSE was Catholic-centric, alongside Christianity in general - so we learnt about the religion's beliefs, and how these are applied to ethical dilemmas (abortion and euthanasia) and how it affects their perception of marriage and the family, and their attitude towards charities (CAFOD, etc), for example. We also learnt about Holy Orders within the Church, and notable people of the faith.
Philosophy&Ethics, on the other hand, wasn't religion-based. The philosophy side concentrated on arguments for the existence of God, while the Ethics part focused on ethical theories and their strengths and weaknesses.
I suppose you have to think about whether you're more interested in the study of religion (so, Religious Studies - although there is probably still a small amount of philosophy in that) or philosophies and ethical decision-making (so more analytical-type stuff). I'd say that Philosophy's probably more on the artsy side (so for English lovers) because there is A LOT of essay-writing, and a lot of science kids can't write an essay to save their lives and there's a bit of pyschology (at least there was in my course content) in Philosophy&Ethics so that'd tie in with one of your other options.
How UCL would look upon it depends what you're going to study, I suppose. Although in the grand scheme of things, they're not really gonna be too fussed between the two (unless you're studying Theology, or something, possibly) and if you're studying something unrelated, then it shows a breadth of interest, which would probably be pretty appealing to them.