Verdant's OK, better than other dry options for British styles, but the vanilla thing is not very typical, whereas 1099 is far more "classic" and is the obvious choice of these two.
No way. As someone who grew up drinking bitter and who likes them on the bitter end - I personally wouldn't go above BU:GU=0.9 just on drinkability grounds, and certainly by going over 1.0 you are wrecking the balance that is key to British styles.
If it needs "mellowing out" then you've brewed it wrong in the first place. The whole reason why AKs became popular with brewers is that after 1880 the tax system changed to one based on the OG of wort, paid at the end of the month in which the wort was produced. This pushed brewers towards Running Bitters that could be turned round quickly as opposed to traditional "IPA"-style Pale Ales which needed aging. This is the beer equivalent of vin de table, quick turnover and knocked back in quantity.
And just look at the BU:GUs of some of Ron's recipes, with the exception of the last two which can perhaps claim war as a justification, they're smack in the usual range for bitter :
0.88
Eldridge Pope 1896
0.83
Russell 1911
0.80
Greene King 1937
0.76
Kidd 1934
0.75
Fullers 1914
0.60
Crowley 1914
0.44(!)
Sheps 1946 (12 IBU in 1.027 but hey, times were tough)
0.30(!!)
Boddies 1914 (what's their excuse?!)