I was advised to go with the 12Kw by our electrician.
Cost was 1,761.
Time Estimate to full completion: 12weeks
However, be aware that your builder/ electrician will need to supply the ECTI cert 2 weeks before the end of that 12weeks.
If you miss the timeline, I think completion is 10 days after receipt of Cert.
This is all just my understanding!
Once you send in the forms they will send you back a quote, which you pay. Then you'll receive a letter with your MPRN.
After this you can also apply for your account with the MPRN#, but I was advised that will be 'matter of minutes' so leaving this to the end..
Hope that helps, good luck, m.