You may well have to replace the inverter (the box that converts direct current from the panels mains alternating current) after 10 – 15 years at a cost of perhaps £800 at 2022 prices. The panels themselves should still produce at least 80% of initial power after 25 years.  

There seems to be a difference of opinion on whether it is necessary to clean panels and you can find advocates for annual cleaning (this could cost £100 – £200) while others say that this isn’t needed in UK conditions (but check more carefully if pigeons roost nearby!). Investigate further before getting this done or trying to do it yourself.