Matt,
A few points to consider before you start the dis board change.
If the property has previously been used as an office the original circuits would have been modified and extended. In my experience this work will not have been done correctly, especially the lighting. You really need to ascertain earth continuity, do a visual check on every fitting and use a multi function tester to get results for continuity and earth loop. As for the sockets you need to make sure you have a ring main, just because you have 2x 2.5 out of the 30 amp fuse you may in fact have two radial circuits due to the ring being broken within the property. I could go on but you get the message.
Are you changing the heating system to a combi boiler? If the second meter is economy 7 you may not need it. It is possible that every room has storage heaters, each on a radial circuit. If you are having radiators you can cut these circuits out of the equation reducing the size of the board you need. Same goes for an immersion heater; you won't need it.
Pay particular attention to the kitchen, kitchen fitters are not known for their electrical prowess, after all we have them to thank for Part.P.
If you are having a new kitchen just rewire it. (you can have all the sockets at the same hight then)
And obviously all those seals on the meters were never there when you bought the property, you know nothing about them, must have been done before.
Also, forgive me for pointing out the obvious, but don't lay any oak flooring or carpets until you are sure all the electrical work is complete.