I bought this one at a large Sainsbury near the hotel, it is not as rare as people imagine. A 500ml bottle goes for around 2 British Pounds, which is the average of other good beers, considering that this one is not, obviously, imported when being purchased in England.
It is a heavy flavored beer, around 4,7% of alcohol and tastes excellent very cold. Before going to London I read a few opinions about it, and some said "it is too bitter", "it doesn't taste like beer XYZ". To say it very bluntly, this beer is for men, if one needs something sweet, go drink juice, honey or water with sugar. The Trooper is dense, goes very well with whatever food you have around and, depending on your resistance to alcohol, 2 bottles of it on an empty stomach will already get you tipsy. I know that myself.
When I stopped at pubs for refreshments while walking around the city, I tried different stuff, other ales unknown to me. The Trooper was saved at the hotel so I could enjoy it in peace and no rush. If you are into beer and consider giving it a try, you will not regret. I searched for it not because I am a beer enthusiast (I am not), but because I will always try a product from one of my favorite bands. If you happen to go to London, my best advice would be visiting any large Sainsbury (not the small ones, delicatessen-style) or directly to Robinsons brewery. Don't waste your time stopping at all those wine/beer shops or random pubs, it is a lottery.
I highly recommend it, as a fan and because it is actually a beer of quality.