I have been raising my ex husbands daughter. I had no children of my own and I love her to bits. She's now 21 and I feel it will be the ultimate show of my love to adopt her. Can this be done in the UK?

I don't know about England but I don't believe you can adopt a 21 yr old they are an adult.
You have already adopted her in your heart and if it is for legal things such as financial and will I do believe you should be able to just include her name on any documents that you care to share with her.

I'm not sure on UK law on this--I really suggest you consult a solicitor.

You MAY be able to adopt her as an adult, with her consent, for LEGAL purposes (making sure she's an heir, next of kin for medical purposes, that sort of thing). However--that sort of law varies depending on where you are, so you really need to see a solicitor about it.