The fines are capped. £1k normal roads £2.5k motorways. I got done doing way over a 30 on country roads near my house by a mobile van and admitted it was me. They sent me a load of paperwork which said plead guilty and you usually get 30% off.
I was issued SP30 with 6 points and a £493 fine which was a significant discount against the £1k cap that I was expecting. There was an admin fee and a victim surcharge on top. Think it totalled at about £700 but by the time I had updated all the insurance etc it was £1k in the end.
Can't give any further advice but best of luck.