Need a mobile SIM/subscription in UK

Wait until (s)he get there and gets information from other students.

Remember that mobiles sold in the UK are "locked" (to the operator that sold them) and are not easily unlocked until they are a couple of years old.

Although unlocked mobiles sold in Belgium have a higher up-front cost, your child could well find that it works out cheaper to have a mobile bought here and to use a UK SIM card when there during term and a Belgian SIM card when here during vacations. Although the quoted roaming surcharges are becoming cheaper thanks to the EC, in practice roaming remains surprisingly expensive.

If you are just looking for a good value UK SIM card it is worth checking out what is on offer from the likes of Tesco as well as direct from the phone companies.

I use a UK sim card in my Belgian mobile as a matter of course when I am over there. Readily available from Tesco and many other outlets and they do not 'lock' any mobile when used.
However, surely the educational establishment where your child will be studying will be able to offer suggestions in advance - the number of oprions available in UK is, quite literally, beyond belief.

many companies offer great deal to you in the UK I personally have an pay and go O2 sim that allows me free calls to Belgium of up to a certain amount each time I top it up a god send when I am over there for a week and my hubbie is back here. If you go into some where like carphone warehouse (phone house here) they will tell you what deals are about and also none of their phones is locked. It is worth to shop around if it's like they will ring people in Belgium they might also want t get a Skype account as that really a cheap way to call.

ps my son is going to Canterbury in the autum too :) to study. I am getting him a dual sim phone so he can keep his old Belgian number and ass a uk sim nto it