When making the load calc 230v nominal voltage is always assumed irregardless of the temptation to use TRMS voltage measured.
Is the ccc of the cable rated from the view point of assumed 20 degrees. Because you mention the cable goes up from the boiler into a void. This will increase ambient temperature and will derate the cable further. As you probably know something like 6242y T&E is rated at 70 degrees operating temperature, which is pretty warm! But is regarded safe up to that temperature. Similarly HOFR is rated to 60 degrees. So a bit warm is not really a problem unless it goes over its rated temperature. As Wilko I would always go for the cable above where it just about works with no margin for derating.