I think people forget just how small our armed forces are at times.
For example the British Army has less than 83,000 soldiers, which means in reality that it has less than 40,000 "facing the enemy in a combat role" soldiers such as Infantry, Artillery, Tanks, Engineers, Air Corps etc with the other 43,000+ being support staff like logistics, admin, medics etc. So imagine if, for example, we suffered a similar amount of casualties as Russia has in the past 26 days of approx 10,000 dead we'd have lost about 1/4 of our "combat" power or 1/8th of our total Army gone in under a month, and the usual rule is for every 1 dead there's 3 to 4 wounded, which would mean we'd potentially lost our entire "combat" power in dead and wounded.
Russia can kind of absorb those sorts of losses which would almost cripple the UK, even if more of their now depleted "elite" units such as the Para's/Naval Infantry take additional time to restock with trained guys, but it's the everyday "normal" infantry, tanks, logistics guys who are being hit the hardest as those tend to be conscripts with minimal levels of training or desire to continue the fight.
