The ATPL remains valid as long as you have a valid Class Rating and medical. This can be a SEP Class Rating as a minimum.
Quite why anyone with an ATPL should limit their employment prospects and airline seniority with a chair-polishing tour is beyond me. The airlines don't give a stuff whether you're a Flt Lt or a Gp Capt - first in is the senior!
The IR can lapse for 5 years before things become tense; less than 5 years and it is included in your Type Rating training for your airline type - more than that and the IR Skill Test has to be with the CAA.
More than 7 years and it's exam time again - if you can find anyone who would actually want to employ you!
See LASORS - all prospective professional pilots should familiarise themselves with it and it's FFFFRREEEEE to download from the CAA website.