Is it legit to turn a dedicated combi bolier supply into a ring circuit with outdoor socket?
There is no rule in the UK preventing you from putting central heating and sockets on the same circuit (which may or may not be a ring). In my experiance a dedicated circuit for central heating is the exception not the rule.

I'm not sure why you would want to make it into a ring, making it into a ring would mean adding an extra cable which would seem to defeat the whole point of this excercise. So it sounds like what you really want to do is create a radial circuit.

The main question that needs to be answered is what size is the cable and how is it installed. Given that what is it's current rating (current rating of cables depends on installation method). This will determine whether you can design a workable combined circuit using the existing cable. Generally for a socket you want the breaker rating to be at least 16A. The breaker current rating should be less than or equal to the current rating of the cable. Volt drop and disconnect times may also be an issue on very long circuits but are unlikely to be a problem in a typical household situation.

If there isn't one already you should install a switched fused connection unit with a 3A fuse to provide overcurrent protection and isolation for the heating system from the shared circuit.

Any new concealed cabling either needs to be 30ma RCD protected or wired using methods that are considered acceptable for non-RCD cabling.

The socket outlets need to be 30ma RCD protected.

If the CU is very new the circuit may already be 30ma RCD protected but on an older install it likely wont be. There are several ways of providing RCD protection.


You can replace the MCB with a RCBO which combines the functions of a MCB and a RCD. 
In a split-load configuration if there is space available in the CU you can move the MCB from the non-RCD side to the rcd side (remember to also move the neutral to the corresponding neutral bar).
You can buy sockets with RCD protection built-in.
You can buy fused connection units with RCD protection built in.
You can fit a din-rail mount RCD in a suitable enclosure (normally you would only do this for higher-current circuits). 


The RCDs i've seen sold as "inline" RCDs seem to be designed for portable use with flexible cables.

Obviously an RCD only protects stuff downstream of it (see comment above about concealed cabling).