Dont Worry you can book wheel chair for your parents so that Air line will do the rest.Immigration,to get in boarding in both transit London and and Baharain.Just pretend that your mum and Dad both are suffering from Artharitis/Gout so they cant walk or have severe back pain(dont have to show them medical certificate) ,and let them know your parents only understand Hindi language while check in ( and I hope you parents do understand or speak at least broken Hindi) to communicate. bear in mind all major airlines and airprts MUST have one Hindi speaking crew/staff either onboard or on the ground coz Dhoti's population forms nearly 25% of the World's total population without hindi speaking staff/crew they cant compete in international tours/travel industry .This is the way my parents travel on their own every year KTM-LONDON return.They board and get off the air craft first before other passengers and get thru immigration counter with the help of wheel chair attendant thru fast lane and their luggage will be claimed by the airline's ground staff and will be handover to your relative at KTM airport who will come to receive your parents.So easy safe and troubleless journey for the elderly is to book wheel chair.GOOD LUCK and wish safe journey to ur Parents.