Electricity (Hazards from Trees) Regulations 2003, regs 9–11; Electricity (Hazards from Trees) Regulations 2003, regs 19–22
Trimming trees near power lines is dangerous, and you should always contact the power company first.
However, legally it’s your responsibility to make sure trees growing from your property keep the right distance from power lines. Power line owners (usually the power companies) can give you a notice requiring you to cut or trim a tree that’s too close to power lines, and they can require you to pay for the work.
You can ask the power line owner for an exemption (“dispensation”) from cutting or trimming the tree, if you can argue that the tree isn’t your responsibility or that it’s unlikely to cause a hazard. If you and the power line owner can’t agree on this, you can ask an arbitrator to make a decision.