How much does a UK barrel of lager weigh in lbs?

I assume that you do not mean the weight of the empty barrel. If so, then the volume of THE BEER INSIDE an Imperial Barrel of lager is 163.65 Liters (or 'litres' just in case you call Britannia your home!). The specific gravity of beers varies by type and alcoholic content from 1.02 for 'light' beers, to 1.05 for lagers and pale ales, to 1.08 for double bocks (Paulaner's Salvator Doppelbock is my favorite). Since a Liter of water (at standard conditions) weighs 1 Kg, the answer is from about 163.65 x 1.02 ~ 167 Kg to 1.63.65 x 1.08 ~ 177 Kg, and for a lager we can just say 163.65 x 1.05 ~ 172 Kg (or 379 lbs.). I've rounded the results since beer is not a standard substance and the specific gravity would vary a bit even from batch to batch. Now, all this beer talk and mathematics (but really, mostly just the beer talk) has made me thirsty, so I think I'll just go to my refrigerator and open a bottle of Bass Ale. Cheers!