The 15-22 has US thread - 1/2 x 28 UNEF.
My advise is buy a SAK Moderator £40. Your Gun Dealer can get it from UK IMPORTERS - JACKSON RIFLES, SCOTLAND.
SAK only do one production run of this Moderator a year with the US thread on it.
Take the old Moderator back to your Gun Dealer to exchange it for the SAK---Job done!!!
Don't bother with the adaptor.