Pay As You Go SIM for Mobile broadband
 

I will be in London for 5 days only and will need mobile broadband for my tablet using a Pocket Wifi with standard sim card.
T-Mobile have a "Pay as you Go" SIM + topup for 10 Pounds... which will give me 5 days @ 250MB per day. (more than enough)
The nearest T-Mobile shop (to where i'm staying) is in Oxford Street.
Can I just walk into this shop and buy the SIM... and will they activate it for me? And will the activation be straight away... or is there some waiting period?
Thank you.

Can't answer your last question but at Heathrow there is a total of nine SIM card vending machines, which offer the following SIM cards: Vendpoint?

The last time I bought a SIM card from such a machine back in 2008 they charged a premium, so it would be cheaper to walk into a shop. Top up vouchers are sold at all the kiosks on the airport.

Thanks for you reply inquisitor.
I'm arriving at Gatwick airport but I imagine Vendpoint machines will be there too.
This Vodafone sim pack was listed and looks good for me: 250mb standard Data sim packs - Lasts up to 1 month - £10 (pre-activated)

As I will continue my travels into Europe, I will also need to buy another pre-paid sim from a T-mobile shop, because they give me the option to buy "Broadband Travel Boosters" eg: 50MB Booster for £10 (30 day expiry).... and is available on "Pay As You Go" which suits me.

Probably a little late, but you can buy Three mobile BB SIMs on Amazon.co.uk for arund 11 pounds for 3GB/3 months - these work a treat - and Amazon does ship internationally.

Thanks UKSTEVE... sounds like a good option to think about (will be leaving for the UK in a couple of months).

There's a 1 GB for 1 month version available as well, there or ebay, if that would be enough.

I just tried to order this... but unfortunately Amazon will NOT ship this to Australia.
So i'm going to have to get one from the Vendpoint machine when arriving in Gatwick.

I could send you the 1 GB / 1 month version

Three SIM £15 Unlim data 300 min 3000 text
 

I just returned from London and Wales and can't begin to say how happy I was with my Three prepaid SIM for one month unlimited data, 300 minutes talk and 3000 texts. Hotspot tethering and Skype were also permitted. Very pleased and very disappointed with restrictive U.S. cell plans. Boo in particular to AT&T.