Hi Anna
have a look on YouTube for “chair based exercise” – I don’t have mobility problems, so haven’t done them, but came across them when I looked last year for someone else. Several half hourish type videos came up that indicated a routine that you could follow.
Amazon has some DVDs too – and simply googling “chair based exercise” brings up lots of resources in addition to the YouTube links.
Also, look at what your local gyms have to offer — City of London has a “Young at Heart” scheme, that is for over 50’s – some of it is standard gym and aerobics, but it also has chair based and gentler exercise; so does the over 55 group at my local community centre.
I just had a look at your profile – if you have provision like this locally, but you’re below the age group specified, it’s worth asking if you can join because of your health issues. Ask your GP if he/she is aware of anything locally too.