See also: Can I legally fit my own wood burner?
As long as your builder’s opening (fireplace recess/inglenook/hole in the wall whatever you want to call it) is made of brick or stone with NO combustible materials (plasterboard is classed as combustible for building regulation purposes but is ignored if in the room on the chimney breast) then there are no Building Regulations that stipulate the size that your builder’s opening should be and no regulations that dictate any air gap around that stove once it is in the recess (although there will be air gap requirements provided by stove manufacturer’s unique to each stove).
If you do have combustible materials in or around your builder’s opening then there are building regulations. These regulations can be found here: distance to combustibles.
There are wall thickness requirements and these can be found in ADJ (Document J of the building regulations). Essentially side walls should be brick or stone and a minimum 200mm thick whilst the rear wall should be 200mm thick or 100mm if back-to-back fireplaces in the same building or if the rear wall is a cavity wall: these regulations were designed for open fires and never updated to take into account modern free-standing stoves. I have never found a builder’s opening that does not conform and rarely does a stovefitter have to worry about wall thickness.
Note: Because of the distance to combustible regulations you can, unless you have a deep inglenook, forget about fitting a wooden beam as your lintel unless you use twinwall flue to protect combustibles (see distance to combustibles). A wooden lintel generally has to be fifteen inches or more away from the flue pipe that leaves your stove. There are ways of reducing this distance using shielding but rarely does it reduce the distance enough and it often is not pleasing to the eye. Again see distance to combustibles.
Note: Because of the distance to combustible regulations you may have difficulty fitting any kind of wooden surround unless you use twin wall flue to protect combustibles (see distance to combustibles). A surround has to be shielded from the flue pipe that leaves your stove or be 15″ or more from it. There are ways of reducing this distance but rarely does it reduce the distance enough and once again it often involves shielding that is not pleasing to the eye. Again see distance to combustibles.
Unless you are going to “use twin wall flue to protect combustibles” then I would suggest you refrain from considering wooden beams or wooden surrounds.
As well as the above many stoves have very stringent requirements when it comes to distance to combustibles (often 60cm or more gap is required).