There seem to be two main concerns here:
1. The flight didn't land immediately after an 'engine failure'.
2. There wasn't an incredible amount of fuel left after eventually landing in the UK (after declaring an emergency).
For the first point, this operator (as many others) does not count a 'contained' engine failure on a 4-engined aircraft as a major emergency necessitating a quick landing. Interestingly, it is quite legal in the USA to take off from an airport you cannot land back at as long as you have an alternate less than 2hrs away.
Once the initial fuss over the surge and shutdown had died away I expect the crew took a objective view of the situation. They carried on in the direction of their flight plan as they were going to be over an area with a reasonable density of suitable airfields for 2-3hrs, should they decide not to continue. They would have calculated the fuel burn at the lower altitude, then considered the impact of another important system failure on the operation, i.e. loss of pressurisation, another engine, etc. This would include MSA & driftdown considerations.
The decision to carry on would have been influenced by reports from engineering as to the severity of the failure and the health of the remaining systems. Crossing the North Atlantic is no different to crossing any other part of the globe where diversion airfields are thin on the ground. (Saharan Africa and Northern Canada, to name but a few.) I would surmise that their flightpath back from the West Coast probably took them fairly close to the 'non-ETOPS' route, leaving them alternates at Goose/Iqualuit, Sondestrom, Keflavik, etc. giving not far away from 1hrs flying to reach an alternate in the event of further problems.
Naturally progressing to the second point, I'm sure that regular checks were being made on the fuel burn and quantity predictions for arrival. I would imagine that LHR had long been ruled out as an achievable destination and that MAN had been selected as the first airfield off the North Atlantic with two independent runways (one of which could be nominated as the alternate for the other). The fact that it was a main maintenance base for the airline and had regular 'shuttle' flights to LHR would have been noted but not allowed to override any other safety criteria.
Just before landing, there was some uncertainty over whether all the fuel from one of many tanks on the -400 would make it to the operating engines. I would guess the crew were 99% sure that it would but just in case, declared a MAYDAY and requested a sterile runway so they didn't have to find out...
They landed with Reserves + some amount. Not an incredibly ususual situation and one which you would expect following a diversion to an alternate in normal operations in any airline. By committing to an 'assured' landing at Manchester they could safely and legally use any 'diversion fuel' on getting to the destination.
I feel that many of the emotional replies on this thread come from those who have a) no experience of modern LR OPS and/or b) are not very familiar with JAR-OPS and it's application...