One of the joys of having an unwritten constitution, such as the one that the UK operates under, is that it can a) bend to new circumstances and b) change in a heartbeat if someone refuses to obey stupid rules.
For the past 400 years, MPs have not had the option of resigning their seat, their only way out was to be disqualified. The honourable way of being disqualified was to accept an office of profit from the Crown. Two such positions, Crown Steward and Bailiff of the three Chiltern Hundreds of Stoke, Desborough and Burnham and the Steward and Bailiff of the Manor of Northstead were available, if applied to, and granted by, the Chancellor of the Exchequer. Otherwise, you’re stuck.
Today Sein Fenn leader Gerry Adams took a third option. Desiring to run for a seat in the Irish parliament, which does not permit sitting Westminster MPs, Adams sent a letter to the Speaker resigning his seat. No Crown Steward position. No Bailiffing. And 400 years of history are kaput.