It is illegal if it means your scooter exceeds the maximum capacity for which you are licensed.  Also, modifications on bikes can invalidate your insurance.

It can be perfectly legal to derestrict your scooter. It depends how old you are.

At 16 you can only ride a Moped (group P) that's 50cc AND 50kph (32mph)

So, derestricted to go faster than 32mph and it's not a Moped anymore.

But :- if you're 17 or over, then you can ride up to 125cc and 14bhp. That's a derestricted moped or learner motorcycle.

If you get nicked doing 40 at 16 you're on a Moped licence it's pretty clear cut; you're done for speeding, then for no licence, so then for no insurance. That's an awful lot of points and a huge fine! Wait 'till you're 17 then derestrict it! Do tell your Insurance company it's derestricted though, you've "modified" the bike and Insurers can be wrigglers when it comes to a payout.

Hope that helps

Nigel