I can see why you want to change from Loadmaster today they are very expensive to replace, however were still good quality. I would want to turn off power likely for more than what a freezer could allow, and I think this is a major factor on cost, if it needs an alternative supply while the job is being done it makes it a lot harder.
There seems to be more than you would normally find in a house, and I wonder how big the supply is? In theory you can use a consumer unit up to 125 amp, but in general 100 amp is the limit for a consumer unit, above that and your looking at a distribution unit not a consumer unit, and that brings the skill of person in charge in question.
I know for use by an ordinary person we should use a consumer unit, but don't know where it says that, so not sure what the official method is when the property needs more than 100 amp?
I did it with one property using a 160A three phase isolator which had three fuses which supplied three consumer units, it was a single phase supply, but since over 100A had to find a way to split supply before the consumer unit and fuse down, the reason why I am saying this is maybe you can't have a single consumer unit?
There were at one time stacked consumer units, but still with a single 100A isolator, after all our efforts at the old mill to allow over 100A the DNO only gave us 100A and said if it was to rupture they would consider upping it so in spite of being a huge property it still worked with just 100A. The figure quoted does seem large for the work, where I live I had two electricians for 6 days and I was charged including parts £2500, can't really see it being a two man job, and £200 a day seems a fair rate, other areas may hit £300 a day, but I would have not expected the work to take 3 days. However using RCBO throughout the price for parts would be high, so it could easy hit that figure with good quality materials, specially if using items normally only used for commercial installations.