It is definitely more efficient to counterfeit larger denominations but you also have to measure the relative difficulties and chance of getting caught.
The old £1 Coin was relatively easy and cheap to counterfeit and the counterfeits were practically perfect (I've had a few over the years, they're REALLY hard to spot unless you know what you're looking for, just Google "spot a fake £1" for some examples of how good they are).
Seems like many of the counterfeit coins come from abroad, in particular countries with a much lower cost of living and where counterfeiting foreign currency may be ignored. There's also a huge market in pushing counterfeit money onto foreign exchanges in other countries.
The old £1 Coin was relatively easy and cheap to counterfeit and the counterfeits were practically perfect (I've had a few over the years, they're REALLY hard to spot unless you know what you're looking for, just Google "spot a fake £1" for some examples of how good they are).
Seems like many of the counterfeit coins come from abroad, in particular countries with a much lower cost of living and where counterfeiting foreign currency may be ignored. There's also a huge market in pushing counterfeit money onto foreign exchanges in other countries.