Like you state it is illegal to run a side plate in the U.K. but again unlikely to lead to any real issues with the law... unless you give them good reason to stop you in the first place
Hi Mike, it's NOT illegal to run a side plate in the UK, and as long you follow some simple guidelines you'll have no trouble getting it through an MOT. If this wasn't the case then half the Harley's would be off the road
1. Regardless of where the plate is mounted the brake/tail lights should be still centred on the bike.
2. The plate should be clearly visible from behind - ie not so far forward towards the pegs that it gets obscured by the rear wheel when viewed from the opposite side.
3. It must be mounted securely and safely (common sense).
4. It must be lit up.
From a UK point of view the problem with the Triumph's own short mudguard was not the side mounted plate but the US spec red flashing indicator/brake lights, which is why the first thing people do over here is swap them for a combined amber/red indicator/brake light usually from Rizoma.
With regards to the Bike Brothers vs Wunderkind short mudguards I think they both look awesome and there's no wrong choice but for me I think the built in indicators of the Wunderkind pip it.
Now whether it's worth doing that's really down to you if you're into Americana then go for it. I think side plates really go well with Apes, drag pipes, and swing arm bags and I wouldn't worry about the Americanisation of a British bike after all back in the day bobbers were an American thing.
Personally while I can see the attraction (particularly if you live somewhere were it doesn't rain) I think in getting rid of the OEM rear mudguard is a mistake, you're removing one of the best design features of the Bobber. After all there are plenty of bike out there with fake hard-tails and short mudguards but there's only the Bobber that has that tire hugging rear mudguard that's so evocative of bikes from the past.