Bit of both of these. It is indeed impractical to get back to the depot and this is probably the best use of the stock, even if it's a bit fiddly.
Space at London Bridge is at a premium but as this train often clogs up other arteries around London and especially also sometimes the junction north of Oxted station, it doesn't always help too much! I can recall a couple of very severe delays which originally stemmed from that shuttle leaving Oxted late, and then if it terminates at East Croydon vice London Bridge (surprisingly common!) the same sort of thing happens there, depending somewhat on which platform is used and therefore what the interlocking is doing.
Some are better than others. I know a couple of staff really try quite hard, though, and it doesn't work, so it's partly lack of being bothered on the passengers' part!
1E10 is often a couple of mins late arriving into LBG due to congestion therefore often leaves late..but has a lot of time to make it up before oxted. Three Bridges still continue to be very skow at clearing signals to a point that you slow down to stop and then have to SG them and then it immediately pings off to green. This loses up to 2mins sometimes and therefore the domino affect. The priority of thameslinks is also odd sometimes as our train couldve been gone and out of way but we are stopped for up.to 5 mins to make way for them.
Also... there has to be a way of running the east grinsteads on time, they often hold us up!!!
It might seem i always have a pop at Three Bridges but its just frustrating. I know they have a tough job.