I am in favour of progressive federalism for Great Britain (I base this on the assumption that the six out of the nine counties that make up Ulster are returned to Ireland).
Scotland can be based on the eight regions with the former Regional Councils reinstated, likewise the same for Wales. England can be based on the existing European Parliament Regions, with the former Metropolitan County Councils of West Midlands, Merseyside, Greater Manchester, West Yorkshire, South Yorkshire, and Tyne & Wear reinstated. Gibraltar can still remain a part of England's South West Region if it wants to (unsure as Gibraltar, alongside Scotland and Northern Ireland voted to remain as members of the EU)
The federal Parliament would be based in Birmingham, so as not to be perceived as too remote from the mainland of Great Britain. Elections would be as follows:
Years ending in 0 and 5: General Elections
Years ending in 1 and 6: Local council elections
Years ending in 2 and 7: County Council/Scotland Regional Council elections
Years ending in 3 and 8: Regional Assemblies, Scottish Parliament, and Welsh Sennedd elections
Years ending in 4 and 9: European Parliament elections (should Great Britain decides to remain or rejoins at a later date)
All of the elections would have a voting system based on proportional representation. For elected members to take up their elected position, their allegiance would be to the electorate and not the monarchy.
Eligibility to stand in any election the candidate would have to have resided for at least five years in the constituency before being considered for selection. This would give the candidate a good understanding of their area and would avoid the parachuting in of candidates into areas they have little knowledge of (e.g. Gorgeous George Galloway falling out with people he works with and turns up as a candidate in areas he has little knowledge of. Also Esther McVey being given the boot from Wirral West in 2015, then turning up in Tatton a couple of years later).
Any incumbent members who get booted out by the electorate will be ineligible to stand for a different election. They would be allowed to stand again in the same type of election that they lost five years later (e.g. unseated at a General Election, it would be the next General Election five years later before becoming eligible to stand again). This would get rid of the careerist politicians who are only in it for themselves (e.g. in the Shettleston area of Glasgow, which has the dubious honour of having the highest amount of bookies and off licences per head of population, Frank McAveety always seems to get booted out of office when being a local councillor, stands for and gets in as an MSP, gets booted out, stands for local council and gets in, gets booted out, stands as an MSP and so on and so forth).
Any incumbent member is found to have caused an act of misconduct while serving office will trigger an automatic by-election. This will give the electorate the chance to decide if they want their incumbent representative to continue in office, or get booted out. For example, the Basingstoke MP Maria Miller was caught with her fingers in the till, and instances of misconduct like this would trigger an automatic by-election.
I did remember starting a thread in this section a couple of years ago on the topic of progressive federalism, and rather than me making a lengthy post even more lengthier, I will leave this post here for the time being.