Constituencies
Constituencies in the Parliament of England were enfranchised (or re-enfranchised in some cases centuries after they last returned members) according to the policy or whim of particular monarchs. By the start of the English Civil War only three of the English constituencies in the Unreformed House of Commons had not yet been enfranchised.
Under the Instrument of Government England (and Wales), Scotland and Ireland were all represented in the First Protectorate Parliament and Second Protectorate Parliament, using a novel scheme of constituencies represented by 1-13 members. The Third Protectorate Parliament reverted to the pre-war distribution (at least for the English members).
By 1660 England, Scotland and Ireland had all reverted to having individual Parliaments.
In the 1670s the last three English constituencies were enfranchised (one two member county and two two member boroughs). For the summary of the final composition of the Parliament of England see the English and Welsh parts of the tables below for 1708–1800 and 1801-1821.
In 1707 45 Scottish members were added to the existing Parliament of England, to form the Parliament of Great Britain. In the 1st Parliament of Great Britain the Scottish members were co-opted from the former Parliament of Scotland, but from 1708 the distribution of members was as set out in the Scottish section of the tables below. Notably all the Scottish seats before 1832 were single member constituencies.
In 1801 100 Irish members were added to the existing Parliament of Great Britain, to form the Parliament of the United Kingdom. In the First Parliament of the United Kingdom some of the Irish members (for constituencies reduced from two seats to one) were co-opted from the former Parliament of Ireland by drawing lots. For the counties and two boroughs (Cork and Dublin) which retained two seats both members continued after the Union.
Read more about this topic: Multi-member Constituencies In The Parliament Of The United Kingdom