Like GWPF, I think the claim that UK offshore wind has genuinely halved in price is dubious, and it is likely that these low-priced windfarm projects will never actually go ahead (quite a few UK windfarm projects, onshore and offshore, have been cancelled over the years for various reasons, and cancellation is not unusual).
My own theory about the halving of the price (when I first heard about this claim in the middle of September) is that it will be connected with some wind energy promotion campaign, and is probably timed to influence the Conservative party conference which starts on October 1st.
After a bit of Googling, I've noticed that Greenpeace is involved in the running of a poster campaign at Westminster tube station, launched by the BBC's Doctor Who actor Peter Capaldi, which is making heavy use of the 50% cost reduction claim:
"Today a huge coalition of companies and civil society organisations including Greenpeace, WWF and the Marine Conservation Society launched a new campaign by taking over Westminster tube station with posters on their walls and ticket gates with the message that electricity from offshore wind is half the price it was just two years ago.
Launching the campaign in Westminster tube station, where ministers and MPs will be walking past striking images of offshore wind farms, Peter Capaldi (Dr Who, The Thick of It, Local Hero) said:
“Great Britain is the world leader in a technology which can generate huge amounts of energy without using any fuel. It’s safe, secure, zero-carbon and economical. In fact it’s halved in price in just two years. That’s 50% off. It’s a great deal. And it may just save the planet. The future of energy in the UK is offshore wind power.”"
The 'huge coalition' seems to consist of Dong Energy, General Electric, Scottish Power Renewables, SSE, Vattenfall, Greenpeace, Marine Conservation Society and WWF.