So if you want to play a little bit with it, we have a few possibilities.
First if its really a O2 phone try this rom:
latest O2 rom
If it works, you have an original NoDo rom installed.
If not, or you want more freedom at you phone do the following:
shut down the phone (e.g. remove battery & reinsert it in the phone)
start the phone & hold the volume down key to enter RUU
Check if the Text at the upper left is mentioned in this thread.
e.g. PD29100 is mine, wich works :)
if yes download an install the xspl from the link and you will get the possibility to flash a HD7 rom you like :)
for information to flash xspl on your phone you need ActiveSync (WinXP) or Windows Mobile Device Center (Vista & Win7)
if your PD number isn't supported at the moment, we need to talk again.
HTC HD7 WP7.8
WeTab 80GB SSD, 2GB RAM, Win8 RTM