There is. The type approval for the car says what bulbs it's supposed to have. Also the minimum wattage the bulbs are supposed to be, which you can't meet using LEDs in a car that came with halogen. It's not road legal but doesn't appear to be enforced unless you take the **** and blind everyone.
The rules are over 30 years old and don't account for LEDs :/
I refer you to my previous statement.

If you put a car through an MOT, with a bulb of a suitable output, colour and pattern, it will not be a problem. The type of light source, main or dip beam aside, is completely moot. The rules do account for LEDs, as they are categorically mentioned in every section. MOT testing regulations are updated on a fairly regular basis to account for such things (DRLs, etc).
You can get high-quality LEDs for main/dip beam purposes that fit in halogen enclosures and put out an excellent pattern, too, without blinding anyone – but you'll pay in excess of £100 for them. I have tested such bulbs, against halogens, and the beam pattern was better and the output less blinding. The only reason they're not legally allowed (if you were unlucky enough to get pulled up for them) is that the regulations haven't moved quickly enough to approve them.
It's just a case of applying a modicum of common sense and making sure they actually work properly in your car's enclosures (case in point: I tried LEDs from the same manufacturer in my sidelights and they were excessively bright and blinding, so ditched them; a generic set of LEDs, however, were much closer to the original spec – but I ended up going back to halogens due to colour issues).
Many just buy the cheapest LED lights they can find, which scatter light in every direction and cause countless problems. I've no doubt that these offer worse illumination of the road ahead, too (particularly given that white/blue light is worse for your night vision, in the real world).