Exceptionally good training.
The US trains well too, but we don't seem to have the romanticism that the UK ascribes to the 'soldier of fiction' profession.
I'm not trying to be anti-American honest but whilst the British Army is quite small in numbers the UK spends far more on training per head compared to the US. Basic Training in the British Army is 23 weeks compared to just 10 weeks in the US Army, and training in specialist units like the Paras and Royal Marines is even longer (Paratroop Regiment: 30 weeks, Royal Marines: 32 weeks). They are very skilled soldiers who are probably also much lower paid than their US counterparts so the prospect of earning £10,000 a month as a military contractor must be hard to resist. I agree though that there is probably a more romantic view of mercenaries in the UK compared to the US.
By the way, have you seen the British Mercs movie "The Wild Geese"? Excellent movie if you haven't.
You're fine, baldrick. We're good.
Comparing US to UK Army training is rather like comparing oranges to lemons... same family, different flavor. The British Para's, for example, have a level of training somewhere between the US 82dn Abn and a Ranger Battalion... same role, but different training standards.
In many ways, I see the British Army rather like the German Reichsheer between Wars One and Two. Since the size of the Army is so small, the troops that are accepted into service are trained to a fare thee well. As one historian put it, "It was an entire army of NCO's." In the budgetary battles at MoD to commission two new carriers and completely upgrade the RAF to F35's, something had to give, and what gave was the Army. Good lord, there are proud regiments in the British Army with service histories stretching back 300 years and reputations that took 1000 men to uphold that have been amalgamated down to under-strength PLATOONS [the entire Scots Establishment is what, 6 battalions including the Scots Dragoon Guards now?] or a rock in a memorial garden [the Glosters, for example].
One of these days, the UK is going to need soldiers, not sailors or airmen, and she's not going to have them. And that's a Goddamn shame. As Grigory Zhukov once put it, "Quantity has a quality all it's own".
As for mercenaries themselves, I live near a major US base with three battalion-equivilents of Special Operations Forces [a Ranger battalion and a Group and a half of Special Forces] and I watched these Blackwater clowns come into town recruiting every single guy they could get their hands on who was within 6 months of the end of their enlistment. They'd offer these huge salaries and large signing bonuses which, as you say, is hard to turn down on a personal level. But what they also did was deprive their units of experienced and trained up NCO's right before they deployed to the Sandbox. More than one SOC unit took unneeded casualties because a needed NCO [like an SF A-team Team Sergeant] was head-hunted away by the Almighty Dollar. Then the poor fool gets his ass whacked working for Halliburton and now his family wants a full military funeral...
As for 'The Wild Geese', yes I've seen it, along with 'The Dogs of War', 'The Mechanic', 'The Killer Elite' and rest of the 'Mercs R Us' movies. Pretty standard barracks movie fare in the 80's. I'm not very impressed with them though. I'm a lifelong military historian and I'm well informed about the shenanigans of the likes of Bob Denard and Mike Hoare. Whenever their type's reputation gets so bad that even Executive Outcomes won't hire them, they pick a Third World country and decide to stage a coup attempt. In the end, I see mercenaries as causing more problems than they solve..