penguin123, as M. Mouse says why is the safest option? The a/c was overweight to make an immediate return to LAX and would have need to dump fuel. This of course takes time and in some cases it can take a lot of time. The crew would only not dump fuel if for example, the a/c had an uncontained fire and it was imperative to land ASAP. Once they had ascertained that the a/c was performing correctly, as they did before leaving the LAX area and was in no danger, electing to continue flight towards the UK presents no great problem. As BahrainLad says statistically having lost one engine you would be extremely unfortunate to lose another and therefore continued flight is acceptable on a 4 engined a/c. Rememeber they were on the west coast of the USA and had many hours of flight over land well known to them with multiple well equiped airfields enroute. This time also afforded them the opportunity to ensure that their Atlantic crossing could be made safely.
As M. Mouse says problems much later on in the flight have brought this to the publics attention. Many of the posters here are professional airline pilots with thousands of flying hours who are basically saying the same thing - continued flight was safe. Please give us all some credit for the jobs we have been trained to do. I'm sure you'd do the same to the solictor carrying out the conveyancing on a house purchase, the doctor prescribing medicines or the mechanic fixing that rusted brake pipe on your car!