Got gap cover through my dealer £25k cover over 4 years for £250... Prob not the best deal out there but gives me peace of mind!
I also looked closely at ala... But a word of warning look carefully at the small print! The ala policy I looked at (back to invoice plus) had a maximum payout 105% of Glass’s Guide retail price... So if the car was written off (depending on spec) it would not cover all options.
You don't need 25K of cover, as stated by others and as the name implies, you only need to cover the GAP between what your insurance will pay you and the cost to replace as new.
In any case I think it's a waste of money, as your car is depreciating all the time if you sold it you would only get market price at the time anyway, you wouldn't expect to get the full new price, so if you get my meaning, you are not really losing anything as such if you don't have GAP cover.