hence lots of 132/33kV substations are referred to as "grid". Does this mean that a 33/xyz connection would be a PT (for primary [distribution] transformer)?
I recall reading in a book about ECML electrification that by the 1980s most 132kV lines were used for regional area distribution rather than their original purpose of national "grid" transmission due to the widespread rollout of the "super grid". The implication being that securing 132kV connections was harder in the late 80s and 90s than in the 1960s because those lines weren't being regularly utilised as much as in previous times, so the imbalance caused by railway loads was more of a problem for the grid operator than in the old days. Of course, not insurmountably - there was something like 15 or 16 original feeder stations on the ECML and only 1 was from a supergrid source back then.
The lines were just as capable. The biggest issue was that loads were increasing with more, faster trains.
It's hard to imagine how low the 1960s electrification loads were, but expresses could be 75mph and trunk routes saw 0.5 or 1tph that today see 3tph each!
As the railway is operated more intensively now, and with faster trains, loads are much higher, increasing the power quality problems.
Supergrid connections were always considered, but costing an order of magnitude more, and only available in a few locations, both economics and operability/resilience meant they were generally a last resort.