Mine's an ICOM 603 but works, I assume, similarly: once ATIS is programmed, you push and hold [H/L] and push [DIAL] to toggle between INT - USA - ATIS.
ATIS cannot be used within British Territorial waters (the UK is not a signatory to the Basle Agreement). The official version is "within 14 miles of the coast" but I suspect that this is 12 miles (UK territorial waters) with a two mile margin for error.
Having studied International Law (some thirty years ago!), I know that Territorial Waters between territories closer than 24 miles (those with 12 mile territorial waters) split the difference and reduce pro rata accordingly. Having studied Constitutional Law as well, however, (also some thirty years ago!) I regret that I am unable to advise (without research and billing commensurately) what the legal position of the Channel Islands is within all this since they are not part of the UK (and I am only qualified in any event to advise on UK law) Phew