Jim:

Diesel trucks are not just for outside anymore. If you've been looking at the UK, I think you will find diesel trucks are used in many applications - inside. Their exhaust emissions can be controlled to make this possible.
Propane units are still popular, but as with all fuel types, carbon monoxide can be a problem if not monitored.
Electrics - over the years are gaining in popularity especially where they are working around food products and close proximity to pedestrian employees.
The cost is more expensive, but when you figure out that you're buying your fuel for the next 5 - 7 years in the initial price then the price becomes more realistic. Coupled with the comparison of a per hour rate for maintenance.
Electrics can come with pneumatic tires and can be used outside, but the capacity has been the problem that causes Companies to look at propane/gas or diesel.
You may also see a difference in cushion tire trucks used in North America and not as popular in the UK.
There are many Companies that sell exhaust purifiers for Diesel units and I'd recommend you speak to some of these people. I can supply an e-mail address for a gentleman in Canada that is very knowledgeable on this subject and could perhaps give you some very accurate information.
Regards
Garry Prosser