That is REALLY pushing it. You will not get a full twin tuner HD PVR under £100 but you can get an A grade Humax HD Fox T2 from their direct sales site for £59 plus postage. You will need to get a USB hard drive and the floods in Thailand have put the price of these up. You could use one retrieved from a redundant computer and put into a USB housing if you have one.
You can record one channel and watch another channel on the same mux - in practice this means you can watch one HD channel while recording another but not, for example, record ITV1 HD and watch ITV2. No problem if you have an IDTV already.
BTW, you may still be able to find the Phillips Freeview+HD box in the stores for under £100. These were made by Pace under a brand licence but the software was done by Phillips in France and was dreadful. It is supposed to be fairly stable after a couple of updates but you would be taking a risk as they do not seem to be supporting it any longer.