This is not the way to proceed in my opinion and the Auto Electrician should know that - this is like flooring over dry-rot.
Either, you bought a Britpart switch and it burnt out because it was rubbish, or there is a fault with the lighting circuits.
I would want to know what exactly is burning out (i.e. which terminals) then test that circuit to see if there is a fault, bad connections can cause higher resistance and power will be dissipated as heat, this can burn out switches, it could be as simple as bad connections. First I'd get the fuse out the fuse-board and measure actual current readings for the current draw on the lighting circuits then do some maths to work out if they add up and fall within expected range, if not you have a current draw somewhere driving up the current killing the switch - as LR used Left and Right circuits it is possible someone inept at wiring has made a mistake in wiring or fixing things and connected everything to the LH or RH circuit and put a bigger fuse in to cope, then weak things like the switch die first.
Another thing could be a bad leakage through other electrical equipment, i.e. your light switch may also be energising a window demister or back-feeding power to some other highish power load. Adding the relay will certainly remove current from the switch but it may be covering up a bigger issue.
