The IOM scheme only covers 75% of the first £20k so, yes, that's £15k maximum guaranteed. It's better than is offered in the Channel Island though where there is no depositor guarantee.
Any account that is called "offshore" in the UK isn't covered by UK rules and guarantees. Similarly anything called "offshore" in the Irish Republic wont be covered by their rules and guarantees.
If it is a significant amount, it may be worth your while to come back to the UK for a period and open some accounts. If doing this, you should try to make sure that you will have a UK address and a utility bill or two in your name to hand. You should also ask the banks/building societies for the form to avoid paying tax that isn't due. Personally I have not encountered problems with an overseas address after opening an account (except for Abbey who stuck a second class stamp on some very important correspondence going to me in New Zealand), but you will need a UK address at the time that you open the account.
The alternative is to look at accounts in other countries. Most European countries offer good guarantees but then they also tend to have rules about only opening accounts for residents. You should also investigate what is available in the country where you work.
My personal preference is to avoid anything that involves the word "offshore" or that is based around a small subsidiary that isn't subject to the same rules as larger organisations.