Thank you!That may be a good plan as Mania said. Even if you marry earlier on into the PMV while offshore, you don't have to apply for the 820/801 straight away. You can always wait until the end of the PMV, maybe a couple of weeks before its expiry. Just be aware of whatever your "must not arrive after" date is if you're going to be offshore for most of the PMV. Then just apply for the BVB to leave again. You could get lucky and get a 6 month one.
Doesn't really matter when you apply as the BVA won't kick in until the 300 ends but if you're offshore for a while after activating the PMV, you can still marry. Just need to get back at some point to apply next stage.