Steala, can you tell us a bit more about the general field you're in? For example, presumably you wouldn't be equally happy with being sent to biochemistry or history?! I am in my second year of a doctorate in Soviet history, so if you are in arts/humanities I could at least offer you my own reflections on the matter..
If you are a scientist, applying for a PhD can often just be like responding to a job advert: someone has a project that needs doing, and they pick the best application for it. In this case your research proposal will obviously be based on the call for applications. There are also a small number of arts/humanities PhDs that work like this (and usually have funding automatically attached as you will be part of a wider project), but the majority involve you putting together your own research proposal, and whether or not you get a place/funding depends on whether the department thinks it is a worthwhile project that could plausibly be done in the time allowed.
If, as I am assuming since you don't mention looking for advertised places, you would be planning to put together your own proposal, I cannot recommend strongly enough that your proposal stems from a fairly specific question that really interests you, and that you put considerable thought into how you might approach it. I speak from experience as someone who applied because I wanted to do a PhD rather than because I felt strongly about a question that I wanted to research! I cobbled together a proposal, was lucky enough to get a place and (I know not how) funding to go with it, and spent a good part of the first year wondering how on earth I was going to get anywhere with it..
As for deadlines, I think with most UK universities it depends how you are planning to fund the PhD. If you are hoping for research council or scholarship funding, the deadline will very likely be in Jan; if you're planning to fund it yourself, many universities carry on accepting applications for places only until they've offered all the places they have, so would theoretically consider apps until the summer. But do check, and obviously the sooner the better!
Long, sorry! But I would be happy to talk about this more if you any of the above has been relevant.