I'd be a lot more impressed by a recipe that won a British competition
- US brewers have some funny ideas about English beers, let alone Scottish ones. I think it's important to emphasise that despite their prominence in the US, strong bitters have almost died out in the UK in the last 50 years, other than in the London market (which is always a bit weird) they tend to be either seasonals or bottle-only specials.
On the one hand, that irregularity means that they've had less "selection pressure" and soin some ways they are much more diverse than regular beers - on the other hand, it often means that the only way to justify small production runs is to partigyle off the main bitter recipe,
Fuller's being a classic example.
I'd certainly insist on brown malt in a porter, but the brown and amber in this beer is taking it away from a straight strong bitter and more into old ale/winter warmer territory. Which isn't to say they won't make a delicious beer, but using appropriate language does help set expectations in the minds of the listener.
If you look at strong ales in the real world, you'll see that historically they could be made with just pale malt and
10% sugar and/or
DME; more
commonly there
would be ~10%
flaked maize in there as well as the sugar, and in the second half of the 20th century you'd find
recipes evolving to include
some crystal although it
could be as little
as 2-5%.
Generally UK brewers have been using far less adjuncts since the 1990s - the Fuller's thread above shows how they used lots of adjuncts in the 1980s version of their partigyle but not now. And I like my stronger beers to have a bit more richness than my session beers, so I probably wouldn't worry too much about invert #1 to dry it out, invert #3 could have a place on flavour grounds (and forget the colour, just use caramel or black malt to tweak the colour - that's what any British brewer would do).
As you can see from the above examples, the BU:GUs are all over the shop from 0.5 to 1.3-ish. Personally I like a decent amount of bitterness and done right this style can "absorb" a fair few IBUs, so I might tweak your bittering a little higher for my personal taste. Nothing crazy, 48IBU would be BU:GU of 0.85.
And even if you were going for a winter warmer, I might knock down your speciality blend to a third for my taste.
I'll say it again - the colour of British beers is meaningless, and you certainly shouldn't adjust taste ingredients to hit a colour target based on commercial beers coloured with caramel.