Heat loss estimates can vary considerably, depending on who is doing them. Have a look at this for a variety of methods including using your gas usage. 

The short answer is yes, there are other ways to check your heat loss estimate. I will post a link tomorrow when I’m at my pc. I would not be surprised if your 6.2-6.4 kW wasn’t far off. The 11kW is the outlier and sounds large based on the type of house. 

Would recommend asking another opinion. The 11 kW heat loss seems high indeed if 2 others came up almost half of that.
I'm also surprised Aira would install a 12 kW heat pump in a property with 6.2 kW heat loss. That's way oversized.

On Octopus, you mention a concern about too small heat pump. An 8 kW Daikin for 6.2 kW heat loss is not too small so that shouldn't be a worry.   

@hcas well spotted - they were going to install a 8KW .

@pie_eater welcome to the forums. If you have any Heat Geeks near you, it might be worth them coming out to do a survey too. 

I did a review of the assumptions of the heat losses and a decent part of it is down to design temp, for Octopus and Aira they use 19 for hallways and bedrooms and 21 bathooms whereas the local company uses 21 everywhere and 22 for bathrooms.

My main question is, how inefficient would the LG 12KW be if heat losses turned out to be 6-7kw ? Would it cost me a lot extra per year or just marginal?

The installers seems to really like the LG 12KW, perhaps because of low noise. Not entirely sure why but he rattled of a range of unsuitable alternatives.

He tells me not to worry it will modulate down as its "inverter driven"

The trouble with heat pumps manufacturer use different criteria for naming the heat pump size. So for actual sizing you need to look at running curves and data.

We spend a very small fraction of the heating season at or below the design heat loss, and a lot at about 6-7 degs which is generally half the heat load. So your 6kW is really nearly 3kW most the year, plus DHW heating. I would allow 3 hrs for DHW heating.

So take you design heat loss of 6.4 and multiply by 24 that becomes 154kWh. Now due to DHW heating you only 21 hrs for house heating, 154 / 21 becomes 7.3kW. So you need a heat pump that can do 7.3kW at -3. But also turn down (min output) ideally to 3.2kW at 7 degs.

My house is a similar size but EPC A, our heat load is 3kW so with your B circa 6kW is way more realistic than 11kW.

@pie_eater

Here is the link I mentioned, it contains various methods of calculating heat loss. It depends how accurate you want to go.

ENERGY-STATS.UK "https://energy-stats.uk/what-size-heat-pump/"

What Size Heat Pump? | Energy Stats UK 2025

What Size Heat Pump do I need? We look at some ideas for determining the correct size for your property.

Using two very simple examples given:

The Heat Geek cheat sheet estimates your heat loss to be between 4.4 and 8.8 kW

Michael Podesta's quick and dirty method gives it as 4.7 kW (-3C OT to 21C IT) using your annual gas figure (assuming that is all heating and not hot water, it would be less if you use gas to heat you hot water).

I certainly would not consider a heat pump of more than 8kW for your property and might well be looking at one in the 5-6 kW range depending on how more detailed calculations come in.

As mentioned before, its important to double check the badged output on heat pumps as that can be misleading and also check how low they can modulate down, Im currently using about 2kW to heat my home (of similar heat loss) and Im starting to cycle.

@bontwoody

ooh that’s a great one thanks. Lots of geeky things to dig into

@pie_eater LOL, you are going to love this forum then! 🤣

Each installer/designer can have their own idiocies 

one wouldnt count our cavity wall insulation because we couldn’t prove it was still working after 40 years (of course not!) so said we needed a12kW Midea NO!

Another used default air leakage despite us having a pressure door test result (said MCS insisted) so recommended a 12kW Daikin NO

I’ve used both heatpunk

HEATPUNK.CO.UK "https://heatpunk.co.uk/loggedout"

Heatpunk

Heatpunk is the free online software for doing full heat loss calculations and designing MCS-compliant heat pump systems, brought to you by Midsummer. Easily specify heat pumps and radiators, automate DNO forms and design reports, and view trade prices to buy instantly from Midsummer.

and PHPP and my own spreadsheet and it comes out as ~7kW HP needed (yep nerd!) 

That corresponds with Podesta rule of thumb well (see link from energy stats above) with our gas usage.

If you ran your gas continuously at the lowest possible temperature you could be sure the gas usage would correspond well.

A heat geek guy did agree that 7kW is what we needed, but even he had to play around with the software since our MVHR system could only be included if the house was built after 2006. Wierd! So that came out as the 7kW Vaillant, so guess which installer we’re going with?