I believe they electrified it on the cheap, but the 333s were able to cope so Skipton workings were able to be 91 hauled eventually.
Lets be clear everything BR did was built down to a cost to get the project passed the treasury that doesn't make it cheap as endless people on this forum refer to BR electrification schemes delivered in the 1980's. The reality was it was electrified to meet the requirements of the day anything more and the treasury would bounce it.
On all the SR schemes the diesel stock needed replacing and the budget wasn't there but because BR traction engineers had improved reliability of the stock more could be sweated from the fleet to cover the electrification schemes and the treasury liked that. In all schemes we were led by the operations department about what level of service they wanted to operate, what type of stock and formations the services would utilise. These factors were then used to optimise the system design which could also cater for N-1 outage of a DC substation and one part of the HV distribution network supplying the substations without impairing the timetabled service. Hastings Line requirements were 12 car to Tunbridge Wells and an 8 car hourly service, half hourly in peak, from there to Hastings. This led us to selecting 11kv for the trackside HV distribution scheme because it was substantially more cost effective than 33kV would have been lowering the cost of the HV distribution system by 40%. We also used XPLE 11kV HV cables, this was first use in the UK, which were very innovative for the time and seen as risky by many established engineers, both within BR and the Area Electricity Boards, but its still in operational use 35 years on.
What BR never foresaw, or couldn't convince the treasury, was the "sparks" effect that electrification schemes would engender that rendered many schemes with inadequate traction power supplies to respond to growing demand. In hindsight Hastings line should have been 33kV as its the 11kV distribution that restricts this line as it would have been a dam site more cost effective than what NR will now be able to deliver the scheme for but that wasn't the name of the game in 1984.
Fortunately the changes to the industry have allowed NR to plan new electrification with the future in mind and secure 25kV supply points with 80MVA capacity that was only dreamed of when we were designing ECML. However, lets not forget that what we did in the 1980's was keep the show on the road that ensured the network stayed intact so its here today and if its needs upgrading that's a measure of success of the industry not one of failure.