"Is there a table..."
There probably is, but I can tell you, baz:
Courses are offered up to (Limit + 10% + 9mph). So, up to 31 in a 20, 42 in a 30, 53 in a 40 etc.
Fixed penalties (£100 and 3 points) are offered up to +20mph in 20 and 30 limits (so, up to 40 and 50 respectively) and up to +25mph in all other limits.
A driver is entitled to decline a course if he wishes (and so be eligible for a Fixed Penalty) and he can decline a FP as well (and so see the matter dealt with in court).
All offences above those limits for the FPs are dealt with in court. When the matter goes to court (either because they are too fast for a fixed penalty or for any other reason) they are dealt with in accordance with the sentencing guidelines:
https://www.sentencingcouncil.org.uk/offences/item/speeding-revised-2017/
You will see reference to "Band A, B and C" fines. Band A is half a week's net income, Band B one week's and Band C one and a half week's. Fines are reduced by a third in the event of a guilty plea. In addition a "Victim Surcharge" of 10% of the fine will be payable (Min £30, Max £170). Finally prosecution costs £85 for a guilty plea, up to £620 for conviction following a trial) will be ordered.
So, compared to even the cheapest of court visits, a Fixed Penalty is good value!