Thats even more than the Avon 300X20 in Vincent spares but at least they have one, next year after Avons close who knows?There is also this:
Coker Classic Motorcycle Front Tread | 300-20
Coker Motorcycle Tires | White Wall Motorcycle Tires Coker Motorcycle tires are available in may popular sizes and styles and are backed by our life of the tread warranty. White Wall Motorcycle tires are available in 5.00-16 and other very popular sizes in this product line. Tread and sidewall...www.cokertire.com
Expensive, especially with shipping to UK - but at least available.