You're paying £200 return to LPL because you waited too late to book. Pure and simple.
The flights have very good load factors on many flights now, and this means the expensive seats are all that is left.
Many passengers sat next to you paid a lot less than this, and twas ever thus.
It was no different in Manx Airlines days. Once the cheap seats had gone, you had to either pick a different flight or dig deep in your pockets.
That's business, in any colour you paint an aircraft/train or ferry.
You can't whinge to the OFT for something like this, unless you want to give them a laugh, life just doesn't work like that.
History is a good starting point. ANY route to/from the Isle of Man that has had two or more operators on it has lost money, sometimes spectacularly. The Catchment area and traffic levels are just not big enough. If they were, the Big Orange and Irish Harp would be sat at Ronaldsway long before now.
The choice is always yours, book early, be flexible on your times and you can get a reasonable price. Any company that has a regular and busy business customer base will ensure the last seats remain available till very late on, for last minute business travel, and the way that happens is with pricing. Always was, always will be.
Sorry about that.