I agree if you leave booking the flight to a month out then you run the risk of the BA flight being more expensive. However if you book further out there is often very little in it. In fact I looked at a travel date of 28th April and the cheapest EZY flight off the island was £94 with BA being £97! You are also quite correct in saying that you can use an Oyster card on LGW to London but not on the Gatwick express so that will mean a very long and extended journey as the train will stop at a lot of stations. So if you want to get back to London at 1am in the morning as the EZY flight has been inevitably delayed and then sat on the slow train then that's fine.
My point is please check these things out as BA is not always the expensive option people think. And please everyone who reads this encourage as many people you know to do so and understand the full implications of the end to end journey travel time.
also a ticket price of £60 which is what BA can be if booked in advance is hardly expensive for an hours flight take a look at the cost of a train journey from London to the North West!