Don't be fooled by the Black Friday Deals offered by the mobile networks, especially if you are on an older RPI contract.
By 'upgrading' to a Black Friday deal, you'll gain CPI + 3.9% annual increases, plus you'll also lose EU (or on Three selected Worldwide) roaming perks.
Typically, the CPI +3.9% annual increases are over 3x that of older RPI contracts.
For example, if your contract is:
Ā£20pm - RPI = 28p p/m, CPi+3.9% = 90p p/m
Ā£40pm - RPI = 56p p/m, CPi+3.9% = Ā£1.80 p/m
Ā£60pm - RPI = 84p p/m, CPi+3.9% = Ā£2.70 p/m
With CPI currently running at a 10 year high, plus if this figure does not decrease by Feb/March next year, you could see a near 8% increase on your bill, in the earlier part of next year. You'll only get the Black Friday deal rate until March/April next year, before the increases kick in.
You should stay on the contract as long as possible, bartering with the network if they try to force you off the contract. If they won't budge on offering a good price, then shift network.
On many networks (especially those that still refuse to split the handset and SIM plan costs), you are better off buying a phone outright from a third party, then going SIM only with the network of your choice.