airtightness of doors with level thresholds

I don't believe that it needs to be a true "level threshold" for building regs purposes because there's a separate main entrance to the house (where there will be a level threshold). I only described it as a level threshold because I didn't want a huge lump on the floor but the difference between 27mm and 15mm will be insignificant.

"At a more fundamental level, why is the boiler room internal? If it has a dirty great hole in it can't it use an external door? For that matter a mud room sounds like something that could be outside the thermal envelope."

The mud room is included inside the thermal envelope because it's also our laundry room and a storeroom. It will have extraction of damp air via the MVHR (due to drying of coats, boots, dog towels, etc.) and some background heating (though at a lower level than other areas of the house).

As currently designed, there is one thermal envelope which includes the main living areas and mud room, plus a separate thermal envelope for the boiler room. The 2 walls and 1 ceiling where these 2 thermal envelopes are in contact will be insulated, so that we don't lose heat from the main living areas into the boiler room. Incidentally, we are planning on putting the accumulator tank in the "living areas" thermal envelope, so that the small amount of heat escaping from the tank remains useful. This is different from any of the other installations we've seen, where the accumulator tank has always been right next to the boiler and consequently outside the main thermal envelope.

You might ask why we're bothering to include a thermal envelope for the boiler when there's going to be a hole in it to allow fresh air to the boiler. There are a few reasons:
1) We've got a bedroom over the boiler room. The 2 walls of the boiler room that aren't in common with the thermal envelope of the main living areas both carry on to first floor level, where those same walls are part of bedroom walls. It's much easier for a builder to use the same construction method for the whole height of the walls, rather than changing the structure halfway up.
2) Long-term, who knows what heat-source we'll end up using? In 20 years time when we replace the boiler, it might be possible to get a room-sealed boiler and therefore the boiler room could become a less-draughty space. If we ended up using a completely different heat source, then the boiler room might be converted into part of the main living accommodation...if the walls weren't insulated it would be far less use.
3) On the heat-loss calculations, we're assuming that the boiler room is at the external air temperature, but in real-life this is unlikely to be the case. Not all of the heat escaping into the boiler room from the main house (or produced by the boiler) will escape to the outside immediately. The insulation in the boiler-room walls will therefore help to keep the boiler room above external air temperature. Reducing temperature extremes is likely to aid the longevity of the boiler

So, I described the boiler room door as "internal" because it links 2 areas that have each walls, floors and ceilings, but it is also "external" because it forms part of a boundary of a thermal envelope. That's why I'm treating it the same as the "external" door from the mud room to the outside.