I'd have suggested L-UK but you're well ahead :)

Our experience is that the cost of with-booking insurance is very low -
I mean that if you don't take it you get very little discount - but it
tends to exclude all pre-existing conditions, both in the traveller and
in anyone else whose illness might affect the trip. In other words it's
as much use to a lupie as a chocolate teapot.

I'd suggest get the insurance you need from wherever you can find it,
try not to pay for the trip-included insurance, and write the cost off
if you have to take it. It does tend to be the way the travel agent
makes their profit on the booking!

Also, start NOW - it'll take you days, maybe weeks, to get approval from
the "medical advisory department".