Western Lord is right. At the time as a young man I worked in the PR team at British Nuclear Fuels plc. The governor was opened up to allow a top speed of 100 mph, if I recall correctly this was so that the kinetic energy of the Peak and carriages would be the same as an HST at 125. I think the deadman's handle was disabled, but there was no power 'control' as such outside: but there was a brake control at frame level on the exterior.
My memory may be playing games, but I'm sure I read an internal document at the time that said the power was to be raised to 100% from within the cab with brakes full on: then the brakes were to be released from the external control. No doubt this would do the traction motors no good at all but it wasn't as if they'd be needed again.
The flask was positioned so that the coupling hook would - depending on sway and hunting - hit the flask's most vulnerable point, being the join between the lid and the main case of the Magnox flask.
That was it - no seats removed, stuff like that. I don't recall anyone saying anything about fuel but I imagine that it had the minimum although in the overall context of things its weight wouldn't have made any significant difference.
Over the years I have seen several conspiracy theorists saying that the test was somehow faked, or the engine mounts were loosened and what have you - and these stories have irritated me because it was all fair and above board, there was no cheating. In fact my ultimate boss at the time, Jeff Preece was there and he said that although he had no doubt that the flask would survive, it was still a gut-wrenching day for him and the CEGB/BR bods on site.