The HTC Sensation XE (together with the Samsung Galaxy S2) can be had from O2 directly for a little as £357 excluding any cashback you might collect along the way.
You will have to forfeit the bundled monthly data allowance which will cut the monthly cost of the package from £13.50 to £10.50 only on a two year contract and there's a £104.99 one off cost to be paid initially.
The deal includes 50 minutes plus 250 texts as well. Think about it as a no-interest two year loan with a deposit of £105. As expected the phone is locked to O2 but you can use it on Tesco Mobile and Giff Gaff and unlocking should be fairly easy.
For what's it worth, the Sensation XE is still a very decent smartphone given that it comes with the expensive urBeats in ear headphones, something that none of the HTC One smartphones seem to have.
It comes with a dual core Qualcomm Snapdragon MSM8660 processor clocked 1.5GHz, 1GB RAM, a bundled 16GB microSD card plus 4GB onboard storage, a 4.3-inch 960 by 540 pixels display, Android 2.3.4 Gingerbread with HTC Sense UI (with an upgrade patch to ICS), an eight megapixel camera with flash and full HD video recording, GPS, Wi-Fi and DLNA compatibility.