Hi and welcome 
You can, yes. There are several countries that might be possibilities for you, which all have different criteria for couples, different children available and different adoption processes. So if you meet the adoption criteria here (you have to be approved by British social services for interncountry adoption before your papers are sent abroad), you need to work out which country you would like and qualify to adopt from.
Most of the information on the web is geared towards Americans adopting, but obviously the process will work differently for you here. I would start with the Intercountry Adoption Centre, and [[http://www.adoptionoverseas.org/]] as well.
Foreign countries will set the same requirements for adopting parents whichever country they are from, so some US based sites will help you work out which countries you qualify for.
When it comes to additional needs, remember that if you adopt a child with these physical special needs who has been in an orphanage for years, you are almost certainly also going to be parenting a child with emotional special needs as well. They are very likely to have difficulties forming secure relationships, may have sensory, social, and/or behavioural issues etc. It's important to research those just as much as DS/heart conditions.
I do completely understand your desire. I chose to adopt two older children (domestically) with emotional and behavioural special needs, and despite how very difficult it's been, I don't regret it at all and there's been plenty of happiness as well. I read a blog of a mum who adopted a girl with DS who had been neglected so appallingly she weighed only 10lb at 9 years old and I'm glad she was adopted. But it's important to go into adoption eyes wide open, and Reece's Rainbow romanticise it IMHO. Also it's illegal in some countries to photolist the children, yet RR are quite happily flouting their laws. However it's a bit irrelevent over here since the process will work differently
And I wish you the best of luck 