Im currently in Lancaster Complex, Southwark Hall.
The majority of halls on campus are ensuites. I know of one hall (Saltash) which share bathrooms and they are grim and are typical hall i.e there can be 20 people per corridor all sharing one kitchen, which may seem cool/socialable but trust me it will soon get on your nerves especially when its four in the morning, you have a 9am lecture and u just wanna sleep. Mill Hall is similar to Saltash but is slightly better because the rooms are ensuite but are very small. Lancaster and Bishop complexes are in my opinion the best. They cost two pound extra a week than Mill and are are only a few years old. All rooms are ensuite and you will never have more than 10 sharing a kitchen. Each flat has 10 rooms and a kitchen nd is behind a locked door so only the 10 people in the flat can enter so security is much better and you dont have hundreds of people traipsing past you door every night like in Mill.
All the flats are sociable and its impossible to know whether you will be in a mixed or same sex flat. The ground floors of all halls are always boys because of safety. i.e. girls getting attacked, which has occured in other london unis I also have a friend whose in an all girls flat, it just luck of the draw. Besides you dont choose which halls to go in you just get allocated them depending on what price bracket you choose. But normally those people who put brunel down as Firm get put in Bishop and Lancaster and the others Mill and similar buildings. There are loads of Halls of residence at Brunel and i have only been in a few but I think the general consesus is that for the extra two quid state a preference for Bishop or Lancaster, they are very nice
They do allocate and there is no way in finding out what halls you are in untill results day when you will be sent an email with your halls details and information in which to pay the deposit.
The chances are that if you put brunel down as firm and choose to pay the highest band i.e. ensuite, you will end up in lancaster or bishop (Y). With regards to mixed sex flats you can never tell what will happen. I have a friend whose in an all girls flat purly because of bad luck but bear in mind some flats need to be all girls (religious reasons) so you may end up filling up an empty room, which is what happened in her place.
Really dont worry about it too much, chances are wherever you are you will have an awesome time