budget airline from your local airport to Barcelona and then the bus to the stadium or one of the organised tours. Have a look on tripadvisor.co.uk for cheap accommodation and the best/cheapest ways to get entry to the stadium.
Prestwick to Reus or Girona via Ryanair is the cheapest I think but you'd still need to get to Barcelona from there (good trains though).
EasyJet fly direct to Barcelona.
Have a look at getting tickets for a game. There's a range of prices and if it's not a big match then they won't be too hard to get. Compare the price to the cost of the tour because I think seeing an actual game would be better.
(I've no interest in football but went with dh to see Barcelona play Seville and have to admit it was amazing)
Easyjet to Barca, bus from the airport to the centre - 4euro I think, then one of the city bus tours that will drop you off outside the stadium - about 15euro for a day, then stadium entry which is about 5 euro?