I have heard both and own the B&W. It is impossible to answer as the speakers are so different. However you should get a good idea auditioning at different dealers simply because they sound so different.
Personally I much prefer the B&W. Also your room size is not so large and the 804s sound fantastic up against a wall in a smallish room, and they are easy to place, sounding good in most situations. They have a very clean bass sound which means they don’t really trigger bass modes in the room, so avoid any boomy bass that other speakers may stimulate in a smallish room like yours. And with your amp the B&Ws will work a treat.
The Tannoys are a more risky investment as they are less tolerant of room size and placement. However you may get lucky with your room/speaker interaction and if you like the more muddled but rich tone of the speaker you may enjoy them more.
If you go with the B&W I thoroughly recommend putting them on Isoacoustic Gaia feet and sit these directly on your floor if you have a hard floor. If you have carpet put the speakers with the Gaia feet on a cheap slab of granite kitchen chopping board. Honestly, this makes a fantastic difference, particularly if your room has a suspended wooden floor (like most older British homes do)
I would always urge testing a speaker in your room first. The speaker/room interaction ultimately defines the sound of the speaker.
