A further thought or two. When I went to work for Inter-City the class 90's were being introduced these loco's had a maximum speed of 110mph obviously this could only be used with a set of Mk3's but if I remember correctly there was a small valve called a DW3 in a DVT just underneith the cooker next to the drivers cab door, It was left to the driver if he wanted to leave it in or isolate it, but when you rode in the DVT cab you could hear the brakes applying and releasing via this valve and the braking was superior. As to the second point the infastructure was not equiped to cater for higher speeds and in the latter years of BR couldn't put up with higher speeds due to restrictions on the amount of money that was spent on it, pity as the ride in a Mk3 was good but when you passed another train at full pelt everthing on your table flew off.
Ian