If it is just a letter, probably nothing. If it's a Notice of Intention to Prosecute (NIP), then they'll first ask you to confirm who was driving, then give you the option of a £60 fine and almost certainly 3 points on your licence, or attendance at a 'Speed Awareness Course' . If you take the course, its a half day and there'll be a fee of around £90, but you avoid the fine and points. At least that's the case in Cheshire where I got snapped doing 38 in a 30 zone (similar circumstance, limit dropped from 'Unlimited' to 30 and I missed the sign).
The larger cost will come when you renew your insurance, I think the points stay on for 5 years, and the accumulated increased premiums will add up to a lot more than the fine, in most cases. In fact I think you're supposed to tell your insurer once convicted.
There are of course, various getouts, of varying legality, and there are lawyers who specialise in challenging these things (did they serve you within the 14 day limit, was the equipment properly sited, calibrated,maintained etc), but probably best just to suck it up.
PePiPoo: Notice of Intended Prosecution (NIP) - UK Motoring Law