EDIT: This has now returned to £39.99. Still a good price, if you want it.
T-Mobile have the Samsung M110 Solid on pay as you go for just £29.99!! This is a minimum of £39.95 elsewhere, as far as I can see (and that currently requires a top-up!). The phone itself is extremely strong, and therefore great for those who work outside, especially in construction industries. Next day delivery is FREE.
With a dust and splash proof rubber casing, the industrial strength Samsung M110 is built for tough workplaces. Its powerful battery gives you a massive 8 hours talk time and 400 hours stand by time so you'll never miss important calls. FM radio means you can listen while you work and speakerphone lets you keep talking even when your hands are full. You can take quick pics on site with the inbuilt VGA camera with a flash that doubles as a torch.
As for the T-Mobile plan, there's lots of little bonuses. If you top-up by £15 in a single Monday-to-Sunday week then they will give you £15 airtime to use the next week. They will also give you a free reward (free texts, minutes or web browsing) every time you top-up, although you have to opt-in by texting "GOODIES" to 441. They also have international calling passes and bundles of minutes, texts and web browsing which work out cheaper than just paying normally. Meanwhile, all UK calls are 20p per minute for the first two minutes each day and 10p per minute for the remainder of that day. Text messages are charged at 10p each.
It's also worth noting that during the check-out process, there's the option to check your local network coverage. DO THIS by changing the "No" box to "Yes" (on the screen after you view the price-plan). If you do this (it's called "Street Check") then you are eligible to return your phone within 14 days if the coverage doesn't meet your needs. There's a note with regards to QuidCo too - they will only pay out if the SIM has been active for 3 months, so don't buy via QuidCo if you don't intend to use the T-mobile SIM.