OK. You have, or are going to have, a granted UK patent. It lasts for 20 years from the filing (not priority) date. How much will it cost once granted?
Renewal fees are due annually by the end of the month in which the filing anniversary lies. (E.g. filed on 3 October: pay renewal in 3 months up to 31 October). They are due for years 5 to 20 (i.e. the first renewal is due 4 years from filing for the 5th year).
As of 6 April 2010, the official fees increase from £70 to £600 for years 5 to 20. Current fees can be checked here. This totals £4,550 over the 20 year term (excluding pre-grant prosecution costs).
If you wish for someone else to monitor and pay the fees (typically an annuity service linked with your patent attorney), increase this cost by about 30%, i.e. ~ £6,000.
Official fees can be cut by half if you make licences of right available (which in term limits enforcement/damages in court). If the cost is a burden ask your patent attorney about this option. Application needs to be made at least 10 days before the next anniversary date to obtain the discount on the next fee.