There is no easy answer to your question, but let me correct your presumptions about the big 2 and 1/2
offshore helio companies.
1. None of the offshore helio companies employs 600-700 pilots in the UK so nobody requires that number of pilots over the next few years.
2. The companies are not desperately short of pilots. They are, however, short of experienced pilots. There are more than enough inexperienced pilots to choose from and, at the moment, they can pick from pilots who have IRs which removes any training risk.
3. The company I work for has recruited alot of inexperienced pilolts recently so slots may be drying up.
My advice is talk to the recruiters in each company and get their advice as they know exactly what the requirements are at any time. The only downside to that is that requirements change daily as contracts are won and lost or pilots lose their licenses for medical reasons.
I joined my present company in 1990 after being offered, at interview, a job starting the next day. I am Ex RAF and had 4500hrs twin time but by the following year the company would not have taken me on as it had enough pilots.
I have talked to several RAF pilots who may be coming on the market soon as they are fed up with flying in the sandpit so competition may get even tougher!
It all comes down to if you want to work offshore you need an IR.
As BRR said luck plays an important role in being selected so you have to keep in touch with the company recruiters.
HF