Comment The money doesn't care about you (Score 1) 735
You want to be only as loyal as makes sense to you. For every story of how workers being loyal has been good I can relate stories of how businesses made decisions that amounted to "sorry, things are bad, had to cut you, and can't afford a dime of severance, wish we could, but bummer" at 4:48pm on a Tuesday.
Depending upon how close you are to finishing a project, you could ask for a touch more time from your new job and then tell your current employer "I can stay 6 weeks if you'd like to finish all this up, if you are willing to pay me at my new pay rate" Don't give them an extra money and give up 800 pounds!
But in the end, companies that wish to staff so thin that they get single points of failure are no different that hosting companies that don't do backups to save money and then expect expect everyone to work crazy hours when their primary storage fails after several years -- they all want everything but don't want to pay for it.