Have you heard of H.F. Holidays, MustangLady? (UK and abroad.)
They have a very comprehensive web site and plenty of holidays to attract you.
They are not specifically for lone travellers but the set up and ethos are perfect for people on their own....house party set up and group walking and excursions, also informal house party type activities in the evenings.....all optional of course.
There are also specials for leisure pursuits, such as bridge, dancing, painting, opera,golf, etc etc.
I have been going on them for 35 years and can thoroughly recommend.