That isn't a warranty then. Have you asked Vaillant or Worcester whether they will still have parts available in 10 yrs time? The manufacturers make much of their revenue from aftersales parts.
Have you asked to see other BG installations?
Have you checked that they will not send a subbie round to install it?
Have have you factored in the £200 you will have to pay BG in years 2-3, 3-4, and 4-5?
This raises their price to £3100 for a job an independent could do for (eg: £2200) less and supply a 5yr manufacturer parts & labour warranty. You would need a service every year (£350 over 4 yrs) to maintain the boiler.
Contrary to what the BG sales patter might have said, BG vans hold virtually no stock parts for these boilers. They have to come out, diagnose the fault correctly, and order the part for delivery to a post office. Two visits and two appointments off work for you. If they get the diagnosis wrong, it's three visits.
Vaillant and Worcester, on the other hand, carry nearly all parts on their dedicated vehicle. They both claim over 98% likelihood of fixing the boiler in one, usually next working day, visit.
Nothing is quite what it seems till you go into the detail.
Check out the difference between the ecoTEC pro and the ecoTEC+ on the web. One is designed with all the enhancements removed for the social housing market to be cheap as chips, the other is for domestic householders.