There is a guy over Bristol predominantly within the migrant communites around Barton Hill, St pauls easton areas offering silly prices, As the guy is somali descent he speaks there language and is readily accepted where as being white I would be seen as an outsider. He offers a board change for ÂŁ200.00 all in, that includes split load rcd and even the certificates! I only worked this out when I did an EICR for a person buying a flat in St Pauls. the paperwork was available next to the board and had test schedule, building control notice and the invoice all attached in the little plastic wallet... it actually tested out okay aswell. only downside was not dressed well inside.
If you buy enough consumer units at once you get a good price but i assume he is not paying tax, NI etc. must have had insurance or a dodgy certificate for the scheme assessment? I looked him up at the time and was on facebook if i recall he offered a 3 bed rewire for 2.2k and had quite a lot of people interested on the page...
Cheapest I have done a board change for is ÂŁ250 and that involved 4 circuits in a small 1 bed flat. Took half a day.