UK pump-diesel is now mandated by law to have , IIRC, 5% bio-content
Somewhat academic really, but that is not correct.
The current legal specification for diesel (EN590) in the UK allows any FAME content between zero and 7%. Makes no difference to the rest of the points made in cockney steve's post though with which I generally agree.
As for using 100LL in classic vehicles, one thing not yet mentioned above is that they may become more difficult to start in Winter (due to the lower vapour pressure of avgas compared to winter grade 4*)