De-catting a car designed to operate with one is going to change the way the exhaust smells - in particular (and I'm assuming here that we're talking about a three-way catalytic converter as those would be the most common) removing the cats is going to increase the levels of Nitrogen Dioxide in the the exhaust gases, and NO2 can have quite a sharp, pungent odor at higher temperatures. Personally I've never noticed it smelling particularly bad on cars without cats but there's going to be some variance on what smells people do and don't consider "bad" - plus to be honest I've never really had the most sensitive sense of smell!
If the car is intended to be made road legal for the UK then you'll have to refit/replace the catalytic converter(s) anyway which likely makes this a moot point. If it's not intended for road use then it's probably something to consider whether you can live with in your intended use of the car.
The "rotten egg" smell that you say you aren't picking up on would be hydrogen sulfide - which you can sometimes get when removing a cat (or where the cat has started to fail), but you're unlikely to get that much, if at all in the UK - the majority of pump-petrol in the UK is either Ultra-low sulfur or even sulfur free, and no sulfur means no hydrogen sulfide.