Alternatively (and probably simpler) would a cooker isolator (switch
only), spurred off a socket on the kitchen ring, feeding the four under
surface sockets for the appliances be acceptable? And could I wire
those sockets in series with a suitable weight of T+E? I'm thinking
that the sequence would be:

Put in whatever switches you like. Provided that the appliance is connected
via either an FCU or a plug and socket, then there is no requirement for
switching.

Just ensure that on a 32A ring, the switch must be double pole and rated at:

one single socket = 13A (20A more likely to be found)
one double socket = 20A
more than one socket = 32A (45A more likely to be found)

Alternatively, a 13A FCU may be used to switch for any number of sockets,
provided the expected appliance load is below this, which it isn't.

Personally, I used a 3 x 20A DP grid switch to switch my tumble
dryer/washing machine and dishwasher independently, all fed off a dedicated
32A radial circuit. I used a junction box to drop the 6mm incoming to 3 x
2.5mm that would fit the switch terminals. As each switch only controls one
single socket which will be fused in the plug, this is permissible for both
short circuit and overloading.

Finally, the cable run mustn't be done in 2.5mm cable after the switch if
the third option (more than one socket not limited to 13A) is used. You must
use at least 4mm, probably 6mm cable, which will be a pig to install, as
getting 2 x 6mm into a wiring accessory will not be easy/possible. You may
have to drop to 2.5mm in a large junction box just outside the socket (which
is permissible as you are allowed to assume 20A max from a double socket or
13A max from a single).

Christian.