The service of the watch at the manufacture, which is excellent, will cost you very close to the market value of the watch itself.
Excellent service, but astronomic price (maybe understandable due to their cost structure, but still astronomic).
On the other hand, finding a skilled / experienced watchmaker who can service these at reasonable cost AND happens to have spare parts in case something goes wrong will be quite hard (I don't know British watchmakers so cannot recommend any).
Therefore, the equation is:
1) if you know such a watchmaker that you trust, go for it
2) if you don't, want the best service and are prepared to spend big £££, go for le Sentier
3) if none of the 2 above, as your watch is looking perfectly fine and if it is running correctly, don't service it.
The only risk is that the watch 'might' wear a bit in time (long time...) and might eventually stop and really need fixing at some point. then, it will be time ot go for number 2)
I would personally only use JLC le sentier service either if the watch is a very special one (a very rare / valuable JLC, or one which means a lot to me) or if the watch is definitively broken and needs parts to be fixed.
In the end, cost of fixing a broken watch in le Sentier is close to equal to cost of servicing a perfectly running watch, and in both cases watch is returned in mint condition.
Therefore I'd better send the watch when it really needs to and not for a 'preventive maintenance' (they will change the critical parts anyway, be the watch broken or not, and cost will be the same)