SMART TRVs are completely Compatible with Pegler Terrier TRV's
You list Pegler Terrier valves as not compatible with Tado Smart TRV's, however, this is not true.
I have Terrier 2 valves installed throughout the house. Like any TRV, the function of the valve gradually deteriorates with age. The ones in my house were over 20 years old and on some radiators no longer worked. The force required to depress the valve had become so great that they would not close off, even with the Pegler manual TRV head.
To remedy this, I have replaced all TRV valves with new Pegler Terrier (3) valves.
ALSO - and this is the main reason for contacting you - because the M30 thread on the Pegler Valve is very 'low profile', the securing nut on the Tado head can foul the nut on the compression joint of the radiator tail piece, preventing the TRV from seating firmly. This was easy to remedy by inserting, a fibre washer to act as a shim between the valve and the Tado head (20mm inner diameter, 26mm outer, 1.5mm thick is perfect).
With these fixes, the system works perfectly.
It is worth noting that I looked at changing the TRVs for a different manufacturer - Drayton TRV4, but found that the force required to move that make of valve was in fact much greater than a brand new Pegler valve, so I opted to stick with Pegler for perfect size match to pipe work, cost and I believe ultimately better battery life.
I hope this information is useful. You just need to advise customers of the requirement to insert a shim for perfect seating (or include one in the adapter pack) and get the customer to check the condition of there valves. If they have deteriorated so badly, is unlikely the valve work effectively even with a manual TRV head fitted.