Crab,
Portland - daytime only.
In fact it's 9 til 9 and that means in the winter months it's not daytime for the whole 12 hours. Perhaps the job they did the other night extracting the crew of the stricken vessel sw of Portland may help you to see that. That job went through to about midnight.
And yes it's daytime only certainly in the summer where there are shed loads of divers and holiday makers around Dorset getting in the ****. A fast response SAR machine such as Portland saves many idiots. If the mil covered this patch poor little johnny would be fish food by the time they got airborne let alone arrived.