Pickles is right about retirement flats not being suitable if your OH with dementia is left on her own. She will be invited to leave if she becomes considered a risk to others in the flats. It makes no difference if you own it.
Also consider whether you would be allowed to have a carer staying overnight for your OH if you may need it - this wasn't allowed in my Mum's flats even for those with a spare bedroom. This wasn't just an arbitrary rule of the company, I know some of the residents felt strongly about it too - they felt that those particular flats were intended for people who could live independently and didn't like the idea of people they didn't know being in the building overnight.
On-site managers or wardens can mean multiple different things. If you are interested in having some support you need to investigate carefully what is on offer at each place individually.
Although Pickles is right that you should think about exit fees, anybody who is asked for these nowadays should be objecting strongly if they are of the unreasonable kind (eg a percentage of the sale value). It shouldn't be necessary to agree to them. See e.g.
http://researchbriefings.files.parliament.uk/documents/SN05994/SN05994.pdf
and if you can get hold of it the report by the now defunct OFT.
I live in a high house price area and saved my mum over £10K by challenging hers.