yes can be sold but the sale is only for the length of time remaining on the lease (max 30 years) there is no automatic right of renewal of the lease that is why it is very difficult to resell leasehold property.
if the application shows it is a genuine tourist visit and not a permanent move you will have no problems. Proof of sufficient funds and evidence your wife has a reason to return to Thailand helps.
Not too difficult if you follow the application process on UK Gov website, no need for an agent. Just involves lots of form uploading and your wife having an interview with VFS Global in Bangkok. Did it for my wife about 6 months ago whole process took about 2 weeks.