over 50 years ago my grandfather had Parkinson's in London. my oh my how treatments for the disease and accessibility have changed since then.
Most pavements now have kerb cuts, all businesses but the very smallest have put in ramps, and many who haven't have temporary ones, all the bigger supermarket outlets have trolleys which can be attached to wheelchairs and provide loaner scooters; even many National Trust properties provide scooters or golf carts. Most lifts (elevators) have wheelchair height buttons. Transport for London ut out an accessibility map and assistance can be obtained for train journeys. All black cabs now need to be wheelchair accessible, as are most buses.
There are down sides, too. Many of the facilities put in at the time of Paralympics in London have not been maintained or improved since (we are on an austerity budget seemingly for the rest of my natural life). While many larger restaurants and hotels with toilets in the basement have disabled facilities on the ground floor, many still don't. Always ask what provision there is - you may be surprised to find that there is accommodation that you didn't expect.
We don't use the term "handicap" in this country, it is seen as offensive by many. Disabled is what we use.
She might invest in a RADAR key (I think they currently are around £3) which will get her into locked public disabled toilets.
For locations like train travel and other places where a human is required for assistance or supplies may be short - like gardens providing transport - a reservation or pre-booking is usually required.
An excellent summary is at https://tfl.gov.uk/transport-accessibility/wheelchair-access-and-avoiding-stairs