There are currently three networks that use O2's mobile phone network - O2, Giffgaff and Tesco - and in theory, they should all be compatible with the Dell Streak which means that you should be able to grab a PAYG Dell Streak for £399 and frolic around with the device.
If you are a PAYG fan and want to use the Streak only to send texts and web, then O2 is offering a Simplicity Text deal (text SIMWEB to 21300) which offers you unlimited cross network texts and web for only £10 a month.
It is also ideal if you plan to use it with the International Friends and Family service which gives you three landline numbers abroad (and 5000 per numbers).
If you don't want to use the IFF package, then hop over to the little known GiffGaff network which is part of O2. their £10 Goody Bag offer touts unlimited internet, texts and 100 minutes.
They do say that unlimited is 100 percent unlimited as long as it is not used for PC use and tethering. To make things even sweeter, their 0800 numbers are free and calls to EU and other selected countries are uber cheap (pennies literally).
If you can afford £20 per month, then O2's 600 minutes with unlimited text and data assuming that you're ready to stick around for one year with that network.
Arguably you could also choose to go for a SIM-Free Dell Streak from Carphone Warehouse for only £429, in which case, we'd suggest that you have a look at our article comparing SIM-only offers available in the UK.