I only have pre Covid experience with special assistance as luckily haven’t needed to use it recently but if you don’t have recent experience yourself it might be worth posting or searching to see if anyone here does. My experiences in the past have been very poor - waiting for an hour on the plane for a wheelchair to arrive (at least I didn’t then have to wait for bags!), the assistance person insisting they would only deliver me to arrivals and not help me the few hundred metres further to the uber meeting point etc. If you can cancel and rebook onto one ticket if would probably make your life a lot easier. If not, I would get in touch directly with special assistance to see if they are permitted to help to that extent landside, e.g. if you have to take the tube / HEX between terminals.