You'd be forever declaring the mod to your insurance company every time you renew. The old question "Has the car been modified from factory specifications in any way". If you answer yes, your premium might go up, if you answer no and are subsequently involved in an accident where braking played a part (most at fault accidents I would assume), they could refuse to pay out because the modified suspension system *could* have had a bearing on the incident and they weren't aware of it.
At worst, 4 new air bags £200 and easy DIY. A valve block kit refurb £15-20 and a compressor refurb similar. Easy to do the whole lot for less than £300 including leads and software (the latter is free). You might well not need bags depending on their condition, but if they are older than 8-10 years or cracked, they should be done.
A home DIYer who has never done them before should be able to do the lot in a day.
Nik