1, A fluless gas fire uses a catalitic converter and these fires cost about 4 times the price !!! Also they cannot be installed in a room of less than 50cubic meters and requires 100 square mm of porpose provided ventilation as well ( kind of defeating the object realy ).
2, If you buy one of the new types of real flame effect fires under 7Kw then additional ventilation is not required. And is a simple swap.
3, Then we get onto the balanced flue fire, these type of fires can only be installed onto an outside wall and should never be fitted to a chimny breast !!!
So i would recomend the second option B&Q sell them they are called Focal Point fires quite cheap aprox �150 and no extra ventilation is required. I have even installed one in my own home & it has been gr8 4yrs old now and no complaints.