Rach/Ed
Sounds fantastic. Best of luck with the venture. I was in Alaska in 2010 and arrived in late July. I spent three weeks there. It was awesome and lovely and warm. My advice would be that you need to have left Alaska by the end of August and I wouldn't be heading to Prudhoe after August. (You might be fine but Alaskan weather is to be respected)
Arriving in Alaska on 20th August would (in my opinion) give you two weeks of good weather and then you'd need to leave, although of course you might get snow free warm weather in Alaska in early September you might not.
You need to weigh up how important this thing is in the UK in August. My experience of travelling and meeting travellers is that many don't give themselves enough time to see the sights and experience things and they rush through. I worry that if you arrive in Alaska on 20th August you'll not have enough time to experience the wonders of Alaska, Yukon and BC. Remember it'll take you a week to get down from Alaksa to south BC and there are things to stop off and look at along the way.
I'd go on 27th July and have a month in the amazing place. You won't regret it.
You could try contacting Phil and Motorcycle Rentals, Tours and Self-Guided Adventures | MotoQuest He lives in Alaska and runs a bike shop (There's free camping next to his shop - run by the Harley shop next door!!) Ask him about the weather in early September - tell him I sent you (Dom) I bought my bike off him in 2010.
If you need some reading material for your trip I wrote a book about my trip - - Gone Riding - | Motorcycling and Volunteering across two continents  and send me a message if you have any other questions which you think I might be able to help with.
The other reason for going early is you could make it down to Nakusp for 21st August and the HUBB meeting http://www.horizonsunlimited.com/events/canwest-2014 I did that and it was great to meet up with people three weeks into my trip.
Dom
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