There are many suppliers who do a tank in tank version, this is in place of an internal coil (if not using the plate X) either a smallish copper coil in the smallish copper UK thermal stores (e.g. McDonald / Albion / Newark etc) or a largish corrugated stainless tube in the largish mild steel European thermal stores.
The T in T is just an expansion of the latter as would be S/S as would contain the potable water. It is normal that this is un-vented i.e. at mains pressure particularly if the primary store side is sealed. It’s then just a UV cylinder inside another cylinder which could be sealed or unsealed.
A ribbed cross section is important not just for extra heat transfer surface but also for additional buckling strength.
There is a comment on a thread here which questions the strength of some of the internal welds within the T in T. Worth searching here for all info on thermal stores / buffer stores
Size wise smallest would be about 600-700 lit then upwards. Remember that the DHW will be about 200 lit so the remaining volume is the buffer volume. As the T in T will be in the upper section the buffer volume will be that below it, so perhaps only 2-300 buffer volume at the smallest size.
Cheers
Mike up North

There are regular references to the 'European mild steel thermal stores', which leaves me with the impression that stores are more plentiful and cheaper on the continent. Can anyone name some suppliers I could check out?

Hi,
Actually, I think you are thinking of the other variant I omitted.
The Akvatherm (me) and Capito tanks use a wrapped up copper coil on a flange at the top. One advantage is that a lower flange can be used instead of solar for a DHW pre heat.
The stainless corrugated coils come in at the very bottom do some tight loops at the lower party, then a wider spiral up to the upper part where they then do tight spirals again. These might hold 30 – 40 or in some cases 60 lit of water so, themselves provide some stored capacity.
Typically, the stainless coils are found from
TiSun
Wagner
Windhager
Viessmann
Kloben
Solar Bayer
Solar Focus
Rotex (also do a PEX variant)
Other suppliers such as Hoval or Boiler Nova and any of the ” PS” range of buffer tanks supplier by say Kotly etc etc with the F designation have a blinded flange this can be used for solar or pre heats of lower and DGHW if upper. Most of mild steel tanks are fabricated in Italy where an awful lot of traditional fabrication is still done.
By the way, many of the T in T are actually mild steel enamelled sometimes called glass lined. You get this in the UK for UV cheaper UV cylinders in place of S/S

Cheers
Mike up North