Frankly, I don't think Flybe is in much of a position to complain, given that it was more than happy, not just to accept a subsidy, but to demand a greater subsidy than the States was willing to give. LHR, as Flybe knows well (let's not forget that before it sold the slots to QF, it tried to sell them to the States), is not "just any old airport", but the world's busiest international airport and it is a hugely important economic boost to the island. The reality is that there are only so many flights a place like JER can get, so it's important that those it chooses are going to deliver the greatest possible economic boost. In other words, it's not so much a subsidy as an investment.
What is comes down to is a realisation that free market economics is great if you're London, New York, Singapore or Hong Kong, but for the likes of Jersey, it needs to hustle and it needs to provide incentives. Flybe can bleat as much as it wants, but it must know the importance of LHR to Jersey.
As for slot sitting, bmi has committed to operating the new route for three years and since the timings for the JER flights are hardly conducive to use on long haul flights (7.10am departure to New York or Mumbai ... anyone?), it should be relatively safe. As long as people use it - and really, that's up to the Jersey punters. If you don't use it, you lose it ...
As to what aircraft bmi is using, who cares? If it's a 50 seater Embraer, I can live with that, as can most business travellers. Given that the 50 seater we've been taking to LHR for the past 7 years takes longer to get to LHR from LGW than the EMB-145 will take to fly from JER, I'm not pushed.