As far as I know, the Uk was a pretty big empire before and during world war 2. Had access to a lot of manpower (already india / african country would have been enough), mineral resources. It had a large industrial sector. It had not received serious damage during the previous wars. It had a large navy for shipments and protection. In short, it was a powerful country.
Still the industrial military production in UK lagged behind the USA and was on par with Nazi Germany. The parity with Nazi Germany is a bit approximated because for sure the UK produced more bombers, but less tanks and so on and so forth, but on average it seems that there was parity. (see N. Kaldor The German war economy )
I would have expected that the UK would have been able to produce more military equipment than the USSR and slightly behind the USA or equal to the USA.
So, why did the UK struggled to produce enough military equipment for its own war effort?
I don't mean this as a be all end all answer, but there was the common issue during the Battle of Britain that Spitfire production was far outstripping the quantity of pilots available, so in that instance at least, production was up to par. Overall, the problem wasn't so much the industrial potential; rather the U-Boat threat actually did rather successfully chokehold Britain as, like you say, much of the raw materials required for war time production had to be imported. This is evident in the severe rationing as even food supplies relied on shipping across the Atlantic.