You're going about this in a way that is slowing you down, and putting the trees at a significant disadvantage.
You are in southern England, correct? If so, your climate is not a problem to grow all of these completely outdoors. Inside they will die--the long thinner trunk are not due to lack of wind, but to lack of sunlight. You may get a season or two more out of them inside, as sequoias are strong growers, but after that you will likely see them start to weaken and then die.
The inside trees are stretching to get at light, that's what happens in darker understory of the forest when they are over topped by much larger trees. The trees outside are more robust because of the better light, humidity and overall growing conditions.
Sequoia were introduced in the U.K. back in the mid 1800s. THere is a large grove of them over in Wales.
You don’t need to go to California to experience some of the wonder that a sequoia grove can deliver to the senses. While they are not yet as big as their American parents, the Coastal Redwoods and Giant Sequoias that can be discovered in Wales at the Charles Ackers Redwood Grove at Naylor Pinetum
onelifeonetree.com
and other sites
Specieswatch In woods across the UK, an imported American stands higher and broader than the trees that surround it
www.theguardian.com
UK firm One Life One Tree is on a mission to plant 100,000 Giant sequoias by 2030 across sites in Buckinghamshire, Hampshire and the Brecon Beacons, which will be Europe's biggest sequoia grove
www.mirror.co.uk
Best thing to do is allow the trees to grow unabated--pruning them now only slows them down with no real benefit. You're after a trunk to work with, not a seedling. To get a decent trunk the tree needs to add eight and top growth, the more top growth, the fatter the trunk. Could take a few years. I'd grow what you have in a container left outside protected from the worst of the winter--if you have temps below 0 C.