Can someone identify the wood for sure ? I think it's oak , the installer did say put that back above the slate as long as the slate projects out further than the timber .....
It's up to you if you deem the building regs important to comply with or overly restrictive but as that beam is just sat there, it could be easily removed, replaced with a cheap concrete lintle and reinstalled after the event.
Oak is very resistant in such situations and countless oak lintles are still in use. But the rules are there for a reason - you might not be daft enough to go to sleep and leave a full woodburner of fuel flat out so that the first flue is glowing red hot - but the next owner might
Alternatively, there are some incredibly convincing fake wood concrete lintles available now (bit pricey though!)