There is no regulation that says a wall must have insulation in the cavity. There are regs that say a wall must reach a certain U-value - which in the old days could have been achieved in different ways other than cavity insulation.
For example, my house was built in 1988 and has no cavity wall insulation. What it does have are 125mm internal thermalite turbo blocks and dry lining. Which in 1988 complied with the minimum required U-Value. At the time external and internal insulation was also widely used and this too would have achieved the minimum U without cavity insulation.
Having said that, U-Values jumped in 1992 and I think that was the year that cavity insulation became popular.
Its generally taken that all domestic properties built after 1930 will be built with cavity walls.
But its not clear cut that your property will be solid wall construction between 1900 - 1930.
In SE England its as likley to have a cavity as not from 1900s onwards. My property was built in 1906 with cavity walls but the same style property 2 doors up built in 1908 is solid wall construction.
In a lot of cases back in the early 1900s it was down to the bricklayer employed on the job.
Easiest way to tell if you can see your brickwork is to check the way the bricks are laid - if they are all long side its a cavity wall, if you have one long side and one end laid its a solid wall.
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