Choices within the UK are
BCFT
Cabair
Glasgow College of Nautical Studies
London Guildhall
Oxford
(Hope I have not forgotten any)
I cannot say which is the best, as I teach Navigation and Flight Planning at BCFT, so would be biased. Contact us all and talk to us, see how you feel about each. Visit if possible, ask about class sizes, cost, accomodation availability, ask to see notes used, get as good a feel for the place as possible. This is important, as the atmosphere has to be one in which you can work successfully.