There is a WIDE version of the Merrel Moab Ventilator available but it seems to be an American import with extremely limited availability in the UK - a couple of odd sizes available on Amazon.co.uk. You may have more success looking on amazon.com
just bear in mind that American sizing is different to UK sizing and there may also be import duty and VAT to pay. It may be worth contacting Merrell in the UK for some advice on UK availablility of the WIDE fitting version of the Merrel Moab Ventilator
https://www.merrell.com/UK/en_GB/content?caid=cs-contact-usThis earlier thread may be worth a read and give one or two pointers to unlined boots available in the UK
http://www.walkingforum.co.uk/index.php?topic=38921.0The Meindl Bernina 2 (Comfort fit) and some of the Hanwag leather lined range available in a wide fit may be worth a look but I would suggest that you will probably need to change the cork/fleece insole in the Meindl Bernina and be prepared to increase your budget significantly if you start looking at leather/leather lined options.
If you are prepared to increase your budget I would highly recommend a visit to Whalley Warm & Dry
https://www.whalleyoutdoor.co.uk/boot-fitting-service/ and I cannot recommend their boot fitting service highly enough. Customers travel from all over the country to use their boot fitting skills and services and are highly recommended by many other forum members. If you ever get up to the Lake District or Yorkshire Dales it's well worth making a detour for a boot fitting appointment.
A simple change of sock may cure your summer heat rash problem as if you aren't affected in the winter you shouldn't really be affected in the summer if you're wearing appropriate socks.