Are there companies for solo holidaymakers?

Yes there are. If you would prefer to travel with other people, there are many ABTA travel companies that offer organised tours aimed at solo travellers.

Activity holidays in particular guarantee that you will be travelling with like-minded people by the very fact that you have all chosen the same kind of trip and are a great way of meeting people and making new friends
 

ABTA tip

You also avoid the dreaded single supplement by sharing your room with a fellow traveller, though this is usually optional not obligatory when booking your trip.