TP10 thermal conductivity 0.022 - 0.023 W/mK Urethane
K7 thermal conductivity 0.020-0.022 W/mK. phenolic

I guess you're aware of this , so does the suggested thickness achieve the design/required u value.
Are you happy with the U value of the cheaper option ?
Can you increase the thickness of the the TP10 to make up for the difference and still see a cost saving ?

cheers Jim

Seems the closer you want to get to 0.15 and below , the more you get hit by the old law of deminishing returns and exponential costs . I'm still not sure if it's worth the additional cost and effort
Personally in your posistion I'd go for the TP10 ( and shop around for a similar cheaper brand)

dovecote,

probably telling you something you already know, but just ensure the reduction in insulation has been taken into account in your heatloss calcs (hence heat input figures - rads/UFH sizing). I work with Scottish Regs which are a bit tougher than in England, but does 0.18W/m2K pass regs for an extension pitch roof?

Just my personal view, but whilst you have the chance, add a bit more insulation of the cheaper stuff. Looks like you're at about 150mm. Add another 20 or 30mm (or more)? Wouldn't change the detailing at the roof that much I'd guess?

Have you tried secondsandco dot co dot uk? They do offer a good saving on kingspan and other named boards, just because there might be a little discrepency in thickness, or mis-aligned printing. Be a little careful with some cheaper brands, as I've sometimes come across large air bubbles as I've cut them, or even obvious bubbles under the foil surfaces - what does that do to lambda values?