How much of a difference will racing in cooler conditions than I train in make?

Hello

I'm currently training for my first marathon in Manchester in a couple of weeks. I live abroad and my training during the summer has been almost exclusively at much hotter temperatures than I expect to race in - even in the evening (when I normally run) it's typically 25 degrees plus and upwards of 60% humidity. By comparison I'd expect race day conditions to be around 10-15 degrees and quite possibly raining (it is Manchester after all).

My question is how much should I factor into my pacing strategy the different weather conditions in the UK? I have heard anecdotally from others out here that they found they could run 30-45s per mile quicker in the UK - but obviously being anecdotal I'm sure this varies from person to person, and depending on the length of the event.

Based on my last 30k I think if I ran the marathon out here at 25 degrees I'd probably run about four hours - but should I be aiming to run quicker in Manchester? My instinct is to be conservative in my pacing strategy (especially given it's my first marathon) and pace it exactly the same as I would here, and if I find that I am comfortable and feel I can lift the pace in the latter stages to do so. However equally I'm fairly competitive and if there is data or broader experience that says that I ought to be able to run slightly quicker I'd like to able to get the best time that I'm capable of.

Any insight gratefully received.

Thanks