It will be hard to find an older model BB free on a contract in the UK. The oldest would probably be the 8100 pearl.
If you really want an older model you could purcahse a second hand one and run it on a pay monthly sim only deal with a blackberry tariff added to it. For example o2's simplicity sim starts at £15 and the Blackberry Bolt on is £10. The other networks have similar deals.
When buying a used device ensure that the PIN has been released from the previous users account or you will have problems setting up your email.
If you were to get a contract go for the latest and greatest
I would suggest a Pearl, curve, Bold or possibly a storm.