They allowed for >250km/h speeds. They were basically competing technology and were developed at the same time as modern high speed rail (and lost).

Not really. Needs its own rails too so you can't use other trains on it or it on other tracks. Regular high speed trains are capable of going faster than UK speed limits anyway so really it would be better to upgrade the existing infrastructure to allow higher speeds if speed was a concern.

A high speed train can also go onto non high speed lines, just at a lower speed. The reverse is also possible although generally you want to avoid blocking up the line too much with slower trains.

Build HS2 and then start HS3-12. Tell NIMBYs to go fuck themselves.

https://youtu.be/3APay0wgYt0

Speed, at the cost of an incompatible and more expensive infrastructure, and even "regular" raillines are now can be upgraded to 300km/h.

Taking the french Aérotrain advantages:

• Higher speed (due to inexistant rail drag)
• More comfortable
• Less noise
• Faster braking
• Way cheaper rails and maintenance (just concrete rails that don’t get rolled on, no steel)
• Rails can be easily elevated, taking less space on the ground and avoiding intersections with roads as well as landlocking. Basically, you can cross fields without bothering farmers too much

https://youtu.be/3APay0wgYt0

No