I assume that in the UK that you have availability of similar products as the USA so the wider wheels here are turf tire wheels. I have a link below to see that tire set
Amazon.com: Kubota BX Turf Tire Set of 4, orange : Patio, Lawn & Garden
www.amazon.com
There is a newer style tire called the R14 that is a combination of the agricultural R1 and industrial R4 (wider) tires. I believe they are what you might be looking at but those might be R4 tires
Kubota Tractor Corp. and its Canadian subsidiary, Kubota Canada Ltd., have picked a tire manufacturer for its first-ever exclusive fitment on Kubota compact tractors...
www.moderntiredealer.com
this is a link to R4 tires
Find many great new & used options and get the best deals for Grass Master 18x8.50-10 R4 Tire for Kubota BX Tractor at the best online prices at eBay! Free shipping for many products!
www.ebay.com
None look like your picture though.
if you’re looking to replace the wheels and tires then you should look into the WHOLEGOODS price list at your dealer in the sales department