Well, I actually also thought the response from CheapElite was rather harsh when first reading it, but anyway, I don't want to get dragged into that.
I also think that you have probably lost your claim to anything else (whathever that may be), for accepting the proposed offer of £80. By the way, what percentage of the room rate was this? I mean did they offer you the room practically for free, was it like 20% off, 50% off or 80% off?
Anyway, a little OT by allow me to share a similar story. I was once booked at the Hilton Brussels Residence for one night. I was due to arrive very late and since the hotel's (mostly a longer-stay property) reception closes after 22:00, passed - as instructed - by the Hilton Brussels to pick up my room key. When I got to the Hilton Residence, I was surprised to see that my "room" was basically the living room of a suite, with the sofabed already made into a bed! I called the 24-hour number and protested at being given a "room" which was not. Manager called me back within a few minutes, saying they were full, no other rooms. I said I could not accept this and they had a responsibility to offer me a hotel room which I had booked and (pre-)paid for. Within half an hour he was there. He had booked me a room at the third Hilton in Brussels (Hilton Brussels City) and had arranged for a taxi to take me there. As it was almost 1:00am and I had a morning meeting, I accepted this. What could I have done? Perhaps asked him to give me the room for free? A certificate for another night's stay? Something else? Anyway, point is, I accepted what he had offered. But then again, just because one accepts a temporary solution, which is clearly below the optimal (=what initially booked) but still within reason GIVEN THE CIRCUMSTANCES, does one forfeit one's right to "full" (or, further) compensation?