we currently have 1 english team playing in scotland, 3 welsh teams playing in england, 1 irish team playing in n.ireland so this will just make it all more formal
at the end of 2010/11 you get the normal final positions and use them to make the usual leagues except championship & div 1, div 2 have 5 relegated rather than 3 or 4 to allow celtic & rangers to join the british championship
the pyramid would be ...

Tier 1
British Premiership
20 teams (top 17 from premiership 10/11 & top 3 of championship)
4 relegated
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Tier 2
British Championship
24 Teams (3 from old premiership, rangers & celtic, usuall 3 promoted from old div 1 & 16 from old championship)
4 promoted (3 up & 1 via play offs)
4 relegated (2 to div 1 north & 2 to div 1 south)
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Tier 3
British Division 1 North
24 teams (10 left from the scotish premier & 14 northernmost from old championships teams who were relegated to make room for celtic & rangers & rest of div 1 & 2)
2 promoted (1 up & 1 via play offs)
3 relegated
British Division 1 South
24 teams (made up from rest of the championship and rest of div 1 & 2)
2 promoted (1 up & 1 via play offs)
3 relegated
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Tier 4
British Division 2 Scotland
18 teams (rest of scotish div 2 & 3)
1 promoted
British Division 2 North
18 teams (made up from northernmost old div 2 & conference)
1 promoted
British Division 2 South
18 teams (made up from southernmost old div 2 & conference)
1 promoted
British Division 2 Central
18 teams (made up from rest of old div 2 & conference)
1 promoted
British Division 2 Wales
18 teams (all 18 from current welsh premier league)
1 promoted
British Division 2 Northern Ireland
18 teams (all 12 of current N.Irish premiership & top 6 of championship 1)
1 promoted
Relegation for the 6 british div 2 teams can be sorted by them in charge