You therefore have a fundamental lack of knowledge of what you are talking about.
Firstly, Tommy's isn't a trauma hospital so won't be accepting any super urgent cases.
Secondly, it's based south of the river, so will have a tiny amount of 'business' coming from an area where very few people live.
Thirdly, protests, demonstrations, London Marathons and other such disruptions are commonplace on Parliament Square so the dispatchers (who are based further south on Waterloo Road incidentally) are familiar with navigating such circumstances.
Fourthly, ambulances and other emergency services are getting through. They never cross the bridge quickly, which if you knew the area you'd understand why.
Now you could have thought, "I don't live there, have no personal knowledge to counter what the ambulance service themselves say, I'll keep schtum to avoid looking silly..." but apparently you quite fancied it today.