Do you need a licence in the UK to keep an owl?

Still checking but you need one to let a captive one free.
The Barn Owl is listed on Schedule 9 of the Wildlife and Countryside Act, which
makes unlicensed releases of captive-bred Barn Owls into the wild illegal. A
licensing scheme for Barn Owl release was introduced 1 January 1993 (Andrew Ward
Associates, 1995). Wild Barn Owls that have been found, taken into care and
rehabilitated can be released under a general licence from DETR.(Department of the Environment.)

Yes , you need an article 10 certificate from DEFRA.
More information:

https://www.gov.uk/guidance/bird-registration

This should not be undertaken if you are not expert in keeping such birds. If you are, there are different ways to obtain an owl including:- Cage and Aviary Birds which has a classified section, a Bird of Prey centre or sanctuary for advice on breeders or the best way is by having a reputable breeder recommended to you. Wherever you purchase your owl you should always get a 'Certificate of Captive Breeding'. The owl should be fitted with a close ring (preferably from the IBR) and/or a microchip and, if applicable, an Article 10 Certificate. Article 10 certificates are only issued for some species of owls. Generally any bird that is found naturally in Europe requires an Article 10. Before purchasing any bird, if you are unsure, contact the DETR to confirm whether a particular species requires an A10 certificate as the legislation can get pretty confusing.

[ Originally from IBR.org, but article no longer online ]

Many thanks for these -I should have been more specific.
Friends found an owl in the forest abandoned by its mother and are taking care of it until it grows with a view to letting it out into the wild again when it is grown up. I don't think they are doing the right thing. Any legalities there?

mibmob - I've just found this on the RSPB site - so this is the legal situation.
Even though you didn't say what the owl was this info would cover all British owls.

'While it is illegal to take barn owls from the wild, they can be kept in captivity if they are injured wild birds undergoing treatment or they are captive-bred. The sale, offer for sale, purchase or commercial use of barn owls is legal only in certain conditions. The birds must be captive-bred and be fitted with a close-ring. They also require a special licence, known as an Article 10 certificate, from the Department of the Environment, Food and rural Affairs (DEFRA).

In addition the barn owl is on Schedule Nine of the Wildlife and Countryside Act, 1981. It is therefore illegal to release barn owls into the wild in Britain without a licence. Licence application forms can be obtained from DEFRA Wildlife Licensing and Registration Section. Applicants must comply with detailed guidelines to help evaluate the effectiveness of captive rear and release techniques as a conservation measure and to discourage poorly planned releases'.

http://www.rspb.org.uk/discoverandenjoynature/discoverandlearn/birdguide/name/b/barnowl/legal_situation.aspx

Consult the RSPCA - they tend to only want to know about emergencies on the phone, so find a centre near you and wander along. Spend some money to help the cause, and ask them about it.

I know someone who has 2 owls he keeps together in an aviary, because they were injured by cars and cannot fly well enough to be released. As I recall he was inspected and the conditions he was keeping them in were deemed satisfactory, but I can't remember by who. Sorry!