Hi,
What great places you have chosen to go, im from north London, near Essex and Barcelona is one of my favourite places! There are some cheap flight options available as both budget airlines, Ryanair and Easyjet, come into Barcelona. Ryanair has some from as little as 12 pounds from Stansted to Bcn Girona airport. If you do decide to come into Girona, you should know that it is quite a way outside the city and you will need to get a transfer bus. This takes just over an hour i think and is not particularly cheap.
Easyjey go from Stansted too and come into Barcelona El Prat, much closer to the city centre. Their flights are a little more expensive- between20 and 30 pounds, but i would say it is a better option. Both airlines fly from other london airports but if you are going from Essex Stansted might be easiest.
It depends what you want accommodation wise- there are some good deals available on rental apartments in the city at this time of year, they often work out cheaper than hotels and give you much more flexibility and your own space. More like being in with the locals too!I dont know if you can add links here but barcelonaflats.co.uk is a good one to try. They are english speaking and you can negociate prices depending on how long you are staying etc.
There are some good little bandb's around and plenty of different hotels. Don't worry too much about location- so many people think they have to be right next to the over priced Las Ramblas, but there is so much more to the city than that and you can get around easily using the cheap public transport. (A 'T 10' ticket is probably your best option..it allows you 10 rides on the bus, metro and tram and is good value.)
I have been to Seville too and it is great...try and see a flamenco show if you can. Look for the ones that look like locals are there and you shouldn't need to pay more than about €10 to see a good one...some are very touristy and overpriced! Also climb up the cathedral bell tower for an excellent 360 degree view of the city. Again the budget airlines fly here too.