i understand I should add a brood box
No, that is not what I suggested.
Running double brood will simplify your work but it is a distant summer target. Instead, use the current two boxes and plan to give the bees the opportunity to make the most of them at some point during spring.
In the next few weeks your colony will expand rapidly as spring nectar & pollen comes in, queen laying accelerates, temps. rise and new bees come on stream. These changes will be determined by the weather, income and the performance of the queen: if your queen is 3 years old and laying slowly, and Wiltshire is a metre under snow by April, all bets will be off.
None of us can know these factors except you, so any advice given here must be digested slowly, the consequences understood clearly, and your considered actions timed with spring development in your area in the coming weeks.
By all means leave your set-up as it is all summer (and add supers to it as needed) but imagine the outcome this spring: as nectar is parked upstairs (its natural location) the brood nest will move down into the BB and all may be accommodated, provided that income does not outpace the rate of brood emergence in the super. If it does, they will dump nectar where they can and congestion may result.
Reversing the current set-up of BB under & brood super above will result in the nest changing from an oval to an hourglass shape. The bees will work to fill the gaps and return the shape to an elongated oval, and if the lower half of the BB is currently empty, which it may well be, they will have a fair bit of space to fill. By giving them this work to do and the queen space to lay, you will have a better chance to avert congestion and so swarming.
The timing of this is your decision (and JBM's comment about brood splitting is valid) but as I said earlier, you must be
reasonably confident of consistently warm weather, and as even our double figures down South will be variable for a while yet, hold your horses. By all means, have a quick check: pull out a frame with care on a warm and windless day, take a few photos to post here, and guesstimate colony strength.
Back to basics: first, heft the hive tomorrow by lifting it an inch at the back with one hand. It's not an exact science: if it's light you'll get a shock, and if heavy you'll say 'bloody hell!' and take off the fondant. Secondly, what is in flower this week in your part of Wiltshire?
