If like mine the two sockets are controlled by same RCD 30 mA type A so you can't turn one socket off without the other, there are a number of types of two gang RCD sockets, passive and active, for a lawn mover etc the active is better, as with a power cut it auto turns off, but with a freezer clearly passive is better.
We also have 10 mA and 30 mA in the main 10 mA are rather expensive so I would use 30 mA.
And we have types, the type AC is no longer compliant so should be type A but typical advert in likes of
screwfix does not say what type, I would not think it really matters type A or type AC as at some point you are likely to fit a new consumer unit with RCD's so only a temporary measure anyway.
The reason I have one is the supply is from the UPS (uninterruptible power supply) for the freezers so it is the one circuit not supplied from the CU (consumer unit) so without the RCD socket it would not be RCD protected. CEF advertises as type A, Tool station say to BS7288:1990 and google says that has been withdrawn, others also refer to BS 7288.
Most sockets with extras, be it a filter, USB outlets, or RCD are limited to 13 amp for the pair, if you can find the data sheet, where a standard twin socket normally 20 amp or 26 amp for the pair. I only have one outdoor unit which gets anywhere near 13 amp and that's the leaf blower.
And nearly every outdoor tool I have is class II, so unless you damage the cable unlikely you need a RCD for outdoor stuff any more than you need it for indoor stuff, I have had my RCBO trip (RCBO = RCD and MCB combined) when my flat roof leaked, i.e. good cause, and my sight impaired mother with Alzheimer's disease put an extension lead in a bucket of water as she thought it was on fire, as the neon flickered. That was reason RCD's where then fitted.
So yes can be fitted, but should not all sockets be protected?