BU:GU is a reasonable approximation for contemporary beers, but have you had your hop utilisation measured? If not, you're only working off taste anyway.
To preserve sweetness you want to mash hotter and use a lower attenuating yeast. However, thick mashes with lots of amber malt won't ferment out very dry anyway, so to switch to Windsor might be over-egging it. If you're fermenting 25 litres of 1080 use a second sachet to get a good ferment going quickly to avoid off flavours resulting from unhealthy yeast and bacteria getting going first. Some yeast-Vit might not go amiss. You won't be able to control final gravity by under-pitching, but you may bring on other faults. That said I used to ferment forty litres up to 1068 with a single sachet of Safale before I had Jamil to tell me (and the people who awarded me prizes) we were wrong!
The risk you run with under pitching on strong ales is that the yeast will be knackered at the end, may fail to condition and will autolyze during long maturation. Do you like Marmite? (maybe) In your beer? (almost certainly not).
There's an old BC article that gives a lot of good advice on brewing strong ales by Matthew Jolly. Matthew's one of the leading lights in the North Cotswold Brewers.
http://www.craftbrewing.org.uk/bcpdf/BC2-5_oct2002.pdf
The stated 1 imperial gallon is just a convention, by the way, to allow you to adjust to your own brewlength. I think most people brewed two or three gallons of strong ales.
, people must have been way hard back then!
