The new ‘England’ regulations prescribe an unlimited fine on summary conviction.
The £100/£50 figures relate to fixed penalty notices which are an administrative resolution.
That is true, and I suppose the same applies to the TfL byelaw (which doesn't mention fixed penalty notices at all - are they elsewhere in the byelaws?).
I'd imagine most enforcement would be via fixed penalty notice though. Was anyone actually convicted under the 'lockdown' regs? (Excluding all the people who were charged wrongly and had their convictions quashed, that is...)