Usually, if its a Police Officer with a handheld speed gun, they are usually looking for people doing excessive speeds, and will have a threshold which is quite a way above the posted limit. For example, on a 50mph road, the threshold could be 75mph. If you're doing below this, even though it could be well above the speed limit, they will just ignore it. If you exceed it however, you will usually be pulled over a bit further down the road and given a FPN/court summons, rather than receiving a FPN in the post.
If it's a speed camera van however, they're usually after people who exceed the limit at all, so they'll be catching people whether they do 55mph or 95mph. Usually you'll receive a FPN through the post within 14 days if this is the case.
That is how it usually happens anyway. You may be unfortunate and receive a FPN for it, but if you do it'll only be £60 + 3 points, and any extra amount that your insurers feel the need to charge you, for the next 3 years. The important thing is to learn from your mistake and not do it again.
If it has been more than 14 days after the offence then it'll be unlikely you will be prosecuted, as the date that the FPN was sent must be within 14 days or it won't be valid.