Plumbing Questions? Ask a Plumber for Answers ASAP
What mechanism are you using for your underfloor central heating, traditionally electric underfloor heating is more effective.
With regard to plumbing in the floor and rads, what is the additional BTU demand on the boiler?
Secondly what size pipe are you planning to T into, as you probably need to find 15mm feed rather than a 10mm tail
A mega flow is a a mains pressured hot water system, so a 40m pipe run is not a problem, but depending on the use eg basin or bath, you may wish to use a 22mm feed for a bath compared to 15mm for basin.
You have already done the hard work for me and confirm your boiler can provide the additional output needed, you may however need to increase the pump speed to circuate the water and re-balance other radiators if you find them not getting hot.
Its not a pressure issue but rather water flow, to confirm your saying the inlet into the mega flow from cold mains is 15mm at a point, or the outlet from the mega flow is 15mm before going back to 22mm, in either case I would look to change it to 22mm if you can, but it rather depends how quickly you want to fill you bath as to if it will cause you any real inconvenience.