When you enter into a contract with any UK network the handset does not form part of that contract. Which is why if you default, the network doesn't chase you for the subsidy on the handset, they chase you for outstanding contractual charges. (the retailer is the one who will chase you for lost commission and handset costs) So when your contract finishes you do NOT have to give them back, they are yours from day one.
I know a lot of people who are moving from Orange, one of the biggest complaints these days is the horrible Orange homescreen they butcher the handsets with. I've moved to Vodafone, with an N70, and I'm loving the phone and the service (For you MS fans, wait til Yahoo! give you YGO, it is fantastic) The Vodafone tariff (?25 a month for 250 minutes and a combination of 1,000 SMS, 333 MMS and 51MB of data) is very well priced in todays market too.