Covering two questions here:

Octopus are coming up with a new special storage heater tariff, called Snug. Not only is the offpeak rate something like 10% lower than BG’s, you get an extra hour during the day at the low rate, too. So it’s worth shopping around.

Now, about Boost. This means different things in different contexts.

1. Some storage heater users refer to a heater function which they call boost. This turns on the fan (if it’s not already on) to blow out warm air, but it can also fire up a separate element within the heater to boost the temperature a bit if the stored heat isn’t enough to keep you warm. 
2. Immersion heaters for hot water tanks often have a boost function. This sort of tank has two elements, the main one at the bottom and a secondary one nearer the top. The first is typically wired to the offpeak circuit, so it is only operative when that circuit is live, e.g. during the night. This will normally heat up the whole tank to its set temperature and then turn itself off thermostatically. The second element is wired to the constant circuit, and is available to heat up the top third or so of the tank if you run out of hot water during the day. Any boost out of offpeak hours will be charged at peak rates.   
3. The meter’s boost function simply provides power to the switched circuit if it’s not currently live. There are usually options for, say, 15- minute, 30-minute or 60-minute boosts. This can be used to supply power to the storage heaters’ charging circuits to heat up the bricks if they’ve gone cold, and if the immersion heater’s main element is also on this switched circuit, it will also have power available during a boost session.

I’m fairly sure that the meters OVO installs (Aclara, L+G) all have this function available, if you know where to look. I would expect other suppliers’ meters (e.g. Kaifa and EDMI) to have it, too.

Just bear in mind that you’re not charged for electricity you use according to what it’s for, only to when it’s used.

​@Firedog and others, can't thank your insights and great explanations enough. That makes sense and yeah basically what i was waffling about! I did enquire with British Gas but they scuppered the plan as we don't have a Quantum Hub fitted to go along with the correct heaters, and isn't a Quantum Water Heater (Elson Zircon direct unvented). Yes the boost for the immersion was more a concern, we try not to use but obviously on high output occasions, we currently heat with an old economy 7 boost box, but that's good to know. Will revisit Octopus with that one there in due course. Cheers all.