Because the operators change their plans every so often, your best bet is to go to each of their website and see what's the cheapest contract there is, and hopefully it's just 12 months. I have not come across Blackberry units sold here (obviously locked to the operator) that are on prepaid (therefore no contract). The following are operators in the UK, in no particular order. Note not all may have BB contracts:
Vodaphone
Orange
O2
T-Mobile
Three
Having said that, there are are sellers like expansys.com that will sell sim-free phones, but the prices are a bit steep. The only reason why the phones seem "dirt cheap" is that the operators subsidize the cost of the phone, and they recoup that cost with lengthy contracts and/or more expensive monthly plans.
Always compute for the TCO (total cost of ownership) to give you the overall true value of what you intend to purchase. For example, if the phone is priced at £100 and the monthly plan is £30 and the contract is 12, then your TCO = phone cost + (monthly plan x number of months in contract), which i this example would give you £460.
Now if the sim-free phone is being sold for more than this amount, then your best bet is to get the contract, not to mention your TCO is spread out over several months so it doesn't seem expensive. If the sim free phone is less than the TCO example above, then it may seem a good deal but your initial cash out will be greater, not to mention the additional cost of having a voice AND data plan added to that.