I wouldn't have considered these to be instalments. For the issuer there is 4 times the administration cost as 4 payments are processed and 4 certs need to be issued instead of 1 (I certainly assumed this was the case anyway?)
I'm not sure of the % of users who pay online versus those who queue or return by post so can't say whether the 12% is justified.
However in general I'd view it as a fee to discourage users paying this way. As for the ability to pay, people choose to have a car so should be aware of ALL the costs associated and budget for them.
A bit of budgeting throughout the year quite easily solves this problem, for example I put aside €125 a month every month to cover my motoring expenses for the year other than petrol, so insurance, tax, rescue service, car service, NCT if needed, as most of these fall due in the same month.