o2 offer their Simplicity Sim only plan, while technically it is a contract the contract is only 30 days (ie 1 month) so you could get a simplicity sim, run it with the unlimited data (going to cost around £21) and cancel the contract at then end of the month (so you only pay for the 1 month you are over here. The issue you would have is they want it paid by direct debit and I suspect they would want UK bank details for that so you would need a friend in the UK willing to basically do the paperwork for you and you give them them cash.
Tariffs - O2 pay monthly SIM only tariffs
Your tariff / Change
Online simplicity 15 SIM Only
Pay Monthly contract
600 texts / 300 mins
Includes Unlimited O2 to O2 Calls £14.69 Your extras
Unlimited Web Bolt £7.34
Monthly cost£22.03 One-off costfree
Quote "Enjoy the flexibility to cancel with just 30 days notice" from the simplicity choice page, you can choose a 1 month contract or 12 month contract, as I read it if you choose the 1 month contract you can cancel it a few days into the contract and the 30 days notice will be the first month you paid for which is all you need?