The IR35 is not a problem as long as you work for more than one employer in 12 months and that this would not be allowed if you were an employee of the main firm you contract to. Also do not take a written contract for a period exceeding 12 months if you are shore working.
Technically every time you go offshore is a new contract so that is not a problem. The other thing to watch is taking perks from the company you are working for. If for example you may have contracted to Sonsub on various jobs but then they offer a bonus of some type which could be deemed to be a staff bonus. That is when you can get into trouble with IR35.
