Brian, you have two choices here really....
You can ask Orange to transfer you onto PAYG, which ends your contract at the point of change. You get to keep your number and you stay with Orange.
Or,
You ask Orange for your PAC code and you then go to any other network of your choice. The PAC code ensures you keep your number.
Something worth looking into is the SIM-Only contracts offered by Vodafone and O2 (I'm sure the other networks offer this too, but I've not looked myself). They are a 30-day contract, are cheaper than the equivalent 12/28/24-month contract and still give you free minutes and messages per month. You don't get a handset as part of the contract, but you can cancel at any time with just 30 days notice and the line rental is cheaper. No need to worry about credit either!
I would then recommend buying a Turbo SIM to unlock your current handset for use on another network. I've just bought a phone that's locked to Orange (Nokia 6500 Slide Black) and the Turbo SIM allows me to use my Vodafone SIM card in it no problem