In order for you to work offshore on a rig, you will need to complete both the MIST and BOSIET courses, irrespective of whether you are a chef or an electrician. The MIST course is a one off and the BOSIET is a 5 year renewal course. You would also need a UKOOA medical for most offshore installations.
COMPEX is obviously desirable because of the nature of the installations and the process.
If I were you, I would look to get your COMPEX first and at least try and seek experience on UK based installations. Also, do not rule out ONSHORE Oil and Gas installations where you will not need MIST or BOSIET courses.
I invested circa ÂŁ2000 on both the MIST and BOSIET courses 6 years ago and never got a job offshore. I spent the past 5 years in Iraq and Africa onshore working on Oil and Gas respectively. I added my COMPEX a couple of years ago when the works changed from early works (accommodation, infrastructure etc) to Plant (LNG Process etc).
In my limited experience, working onshore overseas, an expat will typically hold a supervisory / managerial / inspector / commissioning role as third country national electricians (Indian, Nepalese etc) are much cheaper than yourself.
At the moment, there are very few jobs as the price of oil and gas has dropped significantly over the past 18 months. Don't be put off however, because a lot of experienced workers will have retired and when the prices improve, there will be a demand for workers and there will be a void left by all those who took retirement.
Short version:- get your COMPEX and get into sites in UK soonest and maybe look to overseas onshore. PS:- the money overseas onshore is just as good as offshore......varies from 5k per month to 25k per month!! Don't sell yourself short!
Good luck!!