Since British MP's are not allowed to resign their seat in the House of Commons, any MP wishing to do so must disqualify themselves on a technicality, such as taking an "office of profit under the Crown", for example. One such post is that of "Steward and Bailiff of the Manor of Northstead".
This is one of two positions adminsistered by HM Treasury which have been obsolete since the 19th century but have been retained, (with a nominal salary), purely to facilitate MP's resignation during Parliament. The other is that of "Steward and Bailiff of the Chiltern Hundreds of Stoke, Desbrough and Burnham". Once appointed to such a position, the (ex-)MP retains the position until someone else is appointed or they apply to be relieved of the position.
Having two such positions accomodates the rare occurence of two MP's wishing to resign at the same time.