Hi Sam,
Excellent news on the offer! In terms of en-suite it would depend if you want standard or premium. The best advice I would give there would be to look at your budget.
But for the standard ensuite rooms the most subscribed is often McCombie and Greenow. As they are in the Park Group halls, but also because they are small halls so there generally isn't as many rooms, they also tend to put lots of masters students in those halls. But as you are in early you probably still can apply to them. But I was in Sherfield in my first year and I really enjoyed it. They have quite a homely feel to them, because they are little more traditional, they kind of look like houses from the outside. They are just off campus but its like a 3-5 min walk to campus and 5-10 to the main buildings like the library and students union and doesn't really make any difference at all, and is the same distance as some of the halls that are on campus. Also in the same group is Benyon. They are also popular as they are a little newer than Sherfield, and situated around a lake, the year I was there Benyon was a quieter hall than Sherfield but it does change each year.
If you want a premium ensuite the most popular two are Mackinder and Childs as they quite new and modern (though all premuim rooms are), and near Park Bar which is a large social space, which has a bar, pool tables and a large screen for the sport.
In terms of allocations, I think they do it in the order you apply (kind of like first comes first served sort of thing) so if you are in the first group of applications I would expect you are more likely. Though I can't guarantee that of course, but it is logical.
Hope this helps and feel free to ask me any other questions
Emma
3rd Year Human and Physical Geographer