It seems to me that she would probably not wish to do too much walking. You could argue that London might not be such a good option as it might involve a fair bit of walking about. On the other hand, hiring a car and just driving about areas such as the Cotswolds/ Wales or the south-west - and nipping out of the car to take photos, might be a better option.
Cornwall could still be considered. You will find that many of the small fishing villages such as Polperro, have car parks as you enter them. From these car parks, you walk into the narrow streets of the villages. I don't know what places you actually plan to stay so can't comment on access. (The Travelodge chain can usually provide ground floor accommodation or they have lifts - www.travelodge.co.uk). If choosing Travelodge (or other accommodation), try and ensure that they have free parking outside so that it would be just a few steps for niece to get into building. Downtown hotels usually do not have free parking adjoining their premises.
In the south-west, the A39 between Minehead & Lynmouth would offer a scenic drive without leaving the car. (You could visit the thatched village of Selworthy). Dunster is also worth a visit in this area.
Regarding Cardiff: the city centre is compact, flat & largely traffic free in the main shopping area. The exterior of the Castle can be easily viewed but the Norman keep in the middle would be impossible to climb with the foot broken. Likewise for the other 3 castles that are within 7 miles of the city centre = Castell Coch, Caerphilly Castle & St.Fagans. The first 2 can be easily viewed from outside but inside, they have steep spiral staircases. St.Fagans Castle is set within extensive grounds - which have ornamental gardens & buildings such as barns, farmhouses etc.from different parts of Wales. They have a 'road train' to move people about who have limited mobility. However, to see this place properly, you really need to do a fair bit of walking - and one part has many steep steps up and down a little valley - though the road train has a way around the steps.
Cardiff Bay - 1 mile S of the city centre would be relatively easy if she can walk for about 10 minutes from the car park to the waterfront - which is all on the level.
Also consider driving around Wales - where many roads are scenic - so you could all enjoy the scenery without too much walking. You could, for example, drive to Rhosilli at the western tip of the Gower peninsula (1 hour 35 minutes driving time W of Cardiff). Here, she would still be able to get out of the car and walk a short distance to view the spectacular beach. At nearby Port Eynon & Oxwich, the car parks are almost right on the beach. However, it is the roads in north west Wales which are the most scenic. (The road from Cardiff to Swansea is not particularly scenic and does pass the steelworks town of Port Talbot). In southern Wales,the road up the Wye Valley ( A466) via Tintern (ruined Abbey - easily viewed from car park), is scenic as is much of the A40 in the Brecon area & the A470 running up the middle of Wales.
It might be prudent, if touring about, to know the locations of the main supermarket chains who have free parking and ground floor toilets suitable for 'disabled' access. They are Tesco, Asda (Walmart), Sainsbury's & Morrison.
