i don't think that its gonna make any difference apart from a vosa test. there is a couple in our club with this conversion and never had any bother from police or MOT testers.
This has been covered here many times, it's a very common mod but it isn't actually legal or particularly safe. Under construction & use regs, a rear fog lamp has to be at least 100mm away from any brake light, and therefore converting the reverse lamp in the Mk IV tail light cluster isn't legal as it is within 100mm of the brake lamp. Also a rear fog lamp requires a mirrored less behind it to make it brighter than a standard lamp, therefore simply placing a red bulb in or a red lens in front of the reverse lamp is not sufficient.
Also bare in mind that you actually need to declare any such modification to your insurer, otherwise your policy could be invalidated in the event of a claim, and insurers don't tend to be too keen on insuring illegal modifications to safety lamps.