There's no good reason why the British trench should have no chance of being stolen which is the main issue. It should've just been a trench with a miniscle mp cost for the fact it can be placed anywhere. The fact in can sit in hostile territory and be something troops can easily hop into while the other side needs indirect-fire or flamers to destroy it is not right when doctrinal, even if they're free, trenches do not have this and take up an entire commander slot.
Sure it's protection is slightly worse by being garrison, but it's also harder to kill than Fighting Positions and Bunkers and the occupants inside have no chance of losing models. Most faction-built garrisons kills garrisoned troops, 25% of the time, the Ostheer bunker kills garrisoned troops 100% of the time for lols, while British trenches don't give a damn.
tldr: Allow it to be stolen, done.
Actually there are good reasons why the brit trench is unstealable and it comes with the design of the faction. The two obvious points being that 1. Tommies need cover or a building to achieve their full DPS and 2. The vickers vet 1 requires a garrison of some sort. Since both of these are map dependent the trench is needed to make them worthwhile on all maps. While sandbags can cover part 1, they can also be used by the enemy since brits don't have any engineers to wire off the other side before tier 2.
There's also the fact that building a trench as brits would be an outright liability if it could be stolen. Unlike axis factions brits are sorely lacking in both indirect fire and garrison clearing capability in the early game. Building a trench that could be stolen would ultimately be self harming - why would I want to do that?
Finally I'd also note that of the two ost commanders with (free) trenches, one of them comes with cheap, spammable ostruppen that can keep the trench occupied (and similarly get a bonus from it) whereas anything with a range long enough to use the trench on the brit side costs 280mp.