Yes, as rjm says, you can use any SIM you like. You can put your existing SIM in there, even PAYG.
As long as it is already set up for internet and access, you should be OK.
You'd have to check your PAYG rates for how much you pay to access internet and email.
If you're just using it for voice and text at the moment, 3 have got the best PAYG rates at the moment.
321:
3p minute calls
2p texts
1p MB data
Now, for data, that would work out quite dear, compared to, for example, a Virgin SIM only deal where you get 1200 minutes, UL texts and 1GB for £12/month.
But PAYG rates are always dearer than contract or SIM only rates. It's just a case of shopping around, but the most vital thing before anything else is to make sure you get the level of coverage you need, followed (to my mind) by good customer service, followed by decent rates.
Obviously you may have different priorities. Some folk aren't that bothered about CS, for example. Some folk don't mind paying a bit more to get decent CS, with a call centre in the UK, for example.
As I suggested in another thread, you could get hold of a free PAYG SIM for the carrier you think is best for you, put a fiver on it and bung it in a spare phone to see what the coverage is like. You can also ring CS and see what their service is like.
If you don't have an unlocked phone, it is worth remembering that many mobile providers are MVNO's (Mobile Virtual Network Operators) and they "piggy-back" off other carriers networks.
So, for example, if you have a
Vodafone locked handset, an Asda mobile SIM will work in it.
I can list from memory who piggy-backs off who (as of November 2013), but can't promise this list is complete. If anybody wants to add to or correct it, please feel free.
Asda - Vodafone (But will swap to EE. Current service with Voda will terminate end April 2014)
Sainsburys - Vodafone
Tesco - O2
Virgin - T Mobile
GiffGaff - O2
Talk Mobile - Vodafone.
Lebara - Orange
Family Mobile - T Mobile
Lycamobile - Orange
Talktalk Mobile - Vodafone
Vectone - EE
So, you can see from the list that if you have a Virgin-locked handset, you can use it with the new Asda SIMs (on EE), Lebara, Lyca, Family Mobile, and Vectone SIM cards without unlocking.
If you have an O2-locked handset, you can use Tesco Mobile or GiffGaff SIM cards without unlocking.
However, if you want a SIM-free phone, don't always pay the SIM-free price!
CPW sells most (and I stress MOST) phones unlocked anyway. Even if you buy a PAYG handset on T mobile, they just get a handset and a T Mobile SIM, you pay a tenner to top up and put the two together. But it's generally unlocked. IF IN DOUBT, ASK (DISCREETLY) BEFORE YOU BUY!!!!
I'm not responsible for anyone walking out with what they think is an unlocked handset which turns out not to be!
You can even PAYG upgrade and not pay the tenner top up fee. In the past, I have used a spare O2 SIM card (O2 are the favourite because they allow upgrades) to buy a handset on upgrade. That way, you get it cheaper because you don't have to top up.
http://www.carphonewarehouse.com/mobiles/mobile-phones/SAMSUNG_GALAXY_ACE_2/PPAY?colourCode=BLACK
You can save £30 on Ace 2 by not buying the "Talk Mobile" one.
