The blades come in 2 sizes 35mm and 40mm.
Sounds like you got the 40mm type, would recommend not cutting it down. Best to start again and sell your 40mm key.
I nearly made the same mistake, but the measurment is taken from the tip of the blade to where it enters the chassis, not where it end inside the chassis.
As for cutting in manchester use a timpsons, would try a busy area like train station or centre of town (more likely to be experienced and know what they are doing - commuters and business types passing through). It'll probly cost £5-£7 as you are supplying the key but they dont guarantee it, other people on forums have said its not a problem - Who dares wins !!!