I can't say specifically for Spain, not having lived there, but we've lived in Korea, Japan, Lithuania and Germany so some of that experience may be helpful.
Being from the EU, you should be okay about working in Spain, legally. Language will likely be an issue, unless you're planning on working as an English teacher or in an international school or something like that. If you don't already speak the language, it would be a good idea to start learning now, and then really immerse yourself (take a course if you can, but also listen to the radio, watch TV, hang out in markets...generally listen to real people talking amongst themselves) when you arrive. I hate not being able to communicate effectively with neighbours, shop keepers, and whatnot.
As for getting a flat, that's a bit tricky from overseas unless you're familiar with the city and know where you want to live. We took a trip before moving to Lithuania to familiarise ourselves with the city and figure out where we wanted to live. Before that, we checked rental agencies on line to get an idea of what was available and the prices, keeping in mind that those agencies generally cater to business people and greatly inflate prices. My husband got a tip on a place by word of mouth, and we managed to secure a wonderful old flat in a great Old Town location for about a quarter of the rent advertised by the agencies.
If you can't get to Barcelona before you move, see if you can track down a local paper, preferrably of the "Buy, Sell or Trade" variety that you pick up for free in street boxes and that sort of thing. If you're on a limited budget, you'll find small, cheap flats advertised there. They won't be marvellous, and they might be in dodgy neighbourhoods, so you'll want to do whatever research possible before making a commitment. If I were in your situation, I'd book a hostel for a couple of weeks, and spend the first week or so getting a feel for the city - ask the hostel staff, pore over the classified ads and check out the different neighbourhoods til you have an idea of where you might like to live.
To establish a bank account, it's possible that you'll need proof of a local address, so again that can be tricky to do early on. It's likely that there is a bank in the UK that has a branch in a city the size of Barcelona, so check around and see what they can do about setting up an account for you in advance of your move.
To find out about any possible restrictions on jobs, check with the Foreign Office, (if that's the correct name) and stop by the Spanish Embassy to get as much information - about EVERYTHING - as they can give you. Some embassies tend to be more helpful than others, the Korean embassy in Vancouver gave me stacks of tourist literature about the country before we went, the Lithuanian embassy in Washington DC was rather less informative.
Finally, while you don't have a lot of time before you go, try to get involved with whatever local Spanish community might be close to where you live. This is a great way to practice your language skills, and meet people who may have friends or family who can help you out when you arrive - or just great advice for living in the country. We attended the monthly Lithuanian language Mass for about a year before moving there, and ended up meeting the sister of one of the priests who said Mass (in English) in Vilnius. He was thrilled to meet someone who'd met his sister, and it made an instant connection between us.
Oh, and read. Read whatever you can find about the city, the country, the people, the language - the better prepared you are, the easier it is to adapt in country. James Michener wrote an interesting quasi-travelogue, Iberia
, which is a pretty sweeping background to the whole region. And there are lots more.
Good luck with your preparations and the move!