That's interesting given that it's the only beer named after a hop flavour!
The point is that British beer is all about yeast, water, hops, malt and carbonation all contributing to the whole, each balancing the other without overwhelming them. But I'd agree with you that BU:GU is a useful way of thinking about them, personally my tastes are more northern and so prefer BU:GU in the 85-90% range, but southern beers can be more like 70-75%.
bitter is 90% Marris Otter and 10% crystal malt. Use darker crystal if you want it darker...I haven't brewed extract for years so I have no idea if Marris Otter derived malt extract exists.
Almost no bitter is 10% crystal on its own, in fact commercial ones tend not to go much above 5% without some sugar to balance it out. Fuller's use 7.2% light crystal without sugar
in their main partigyle and to my taste that's about as high as I'd want to go, a lot of northern bitters only have 2-3% (or none at all). There may be commercial examples out there using 10% crystal with no sugar, but I'd regard them as extreme examples.
Don't use "flavour" ingredients like crystal to colour beer - one reason North Americans end up putting stupid amounts of crystal in bitter is that they fail to realise that most of the colour is added via small amounts of caramel or black malt. Use the amount of the right crystal you need to get the flavour right, then either don't worry about the colour or use colouring agents. Look at these
photos of Landlord - it's paler than some southern ones but it gives you some idea.
I've certainly seen Otter extract in the UK, but it's not common, and frankly I don't think you'd see much advantage. Remember that Otter is a pretty rare variety of barley in British brewing, it's loved by homebrewers and some microbreweries but most commercial brewers aren't prepared to pay the premium for a 60-year-old variety that is horrible to grow.
I have some East Kent Goldings and Fuggles and wanted to try to create an ESB, preferably something close to a Kentish ale.
If the requirement is for a strong bitter (ESB is a specific beer from Fuller's, not a style) and Kentish then Bishop's Finger is the obvious benchmark. We know from
Shepherd Neame's allergen list that it has barley but no wheat or maize. Anecdotally their core beers use only pale malt (ordinary, not fancy stuff like Otter), crystal and syrup. So it's probably something like pale malt with 8% crystal and 10% syrup.
I believe Bishop's Finger has Target for bittering to 40 IBU and Goldings as late additions, say an ounce at 10 minutes and an ounce as dry hop. Ah - for 3 gallons maybe reduce that a bit, but you can't really go wrong with Goldings as a dry hop in any quantity... But for a generic southern English ale you can't really go wrong with Fuggles (although it's not a personal favourite) in there somewhere, and it's not essential to use a British hop for bittering if you have something higher alpha kicking around.