How do you need the £2500?
If it's for a purchase that can be put on a credit card, the most efficient way would be to go for a 0% on purchases card. Then you'd pay no fee at all.
If you need "cash" or equivalent (ie you need it in your bank current account), then you need a card that would allow you to do a transfer to a current account or give you credit card cheques. Typically there is a one off 3% fee. Many cards have introductory deals, so apart from the 3% you wouldn't pay interest for a set period - typically 6 to 12 months. My Halifax card came with such a deal - including credit card cheques.
One problem is the amount. The best deals are on "new" credit cards. But unfortunately initial credit card limits can be quite low. You could apply for more than one a time. Repeated searches can be a problem, but a couple should be OK. Also if you do them at the same time you're in a better position than getting one set up and then applying for a second. (Because at the time of the second application, the credit limit + balance on the first will be visible.)