Angie
I haven't done a London disability page primarily because I don't feel I've spent enough time there to qualify as an authority.
But a couple of things - 1 do not I repeat do not tell mom to leave her scooter at home. If it is one of those light weight ones then by all means have her bring it. If it isn't have her rent a light weight collapsible here in US.
Most of the London Tube system is inaccessible. The good news is all London cabs are required to have a lift - the bad news? about 1/2 the drivers don't know how to use them. Victoria Station has some problems but you should be able to manouever your way through the station. There is a wheelchair assistance office and they can be extremely helpful.
The british museum is accessible but somebody has to go in and they will direct you to the accessible entrance.
One of the hardest things for people with foot problems is lifting themselves off low toilets. If your home doesn't have a tall toilet (and most don't) you can purchase a toilet seat riser (available in the medical supply section of boots). Here in US about $20 so that's about 10 pounds.
The boats that go up and down the thames have a few that are accessible. You'll have to check on times. The London eye is wonderfully accessible and they offer a discount for the wheelchair person.
Tower of London was inaccessible in 2004. Don't know about now.
Most of the theatres have some accessible seats. I seem to remember the exception to that was the place playing the Complete Shakespear in 2 acts and the complete history of the world in 2 acts. Don't even know if that is still running. Lion King was accessible.
Most of the streets have cutaway curbs so getting around isn't going to be terribly difficult.
And once again - to make the visit the most comfortable for your mother in law have her bring her scooter. The thing that is hardest for those of us with disabilities is feeling like we are a burden. Having people push us in a wheelchair definitely defines being a burden.
Hampton Castle and Brighton are amazingly accessible. Hampton actually has a power scooter you can rent.