Depends how good the drivers are at keeping up with ATO performance! If they can match it, then the implication is nothing. If they can’t, and bear in mind it only takes one to pull back everything behind it, then it would restrict line capacity and ultimately make the timetable undeliverable if there isn’t enough slack to make up the headways. Same is seen on the LU lines. Some drivers are capable of matching ATO performance. Most aren’t, so you end up with one train holding back everything behind it - gaps develop, trains don’t present at junctions at the right time so conflicts arise causing knock-on delays, and ultimately the timetable then relies on recovery time, which may not exist at busy times of day. It also depends on factors like driving policy and how the ATO system is designed. On the Jubilee and Northern lines overrunning a limit of movement authority *isnt* regarded as a safety-related incident, unlike the Central and Victoria lines where it is, so giving more freedom to drive around at or near the limits of permissibility / performance. The Thameslink setup I presume is more like the latter, then there’s factors like defensive / professional driving policy to consider and what this allows the driver to do or not do.