The snappily-named Ecodesign for Energy-Related Products and Energy Information Regulations 2021 (which provide a 'right to repair') only apply to welding equipment, refrigerating appliances, dishwashers, washing machines, washer-dryers, electric motors, variable speed drives and electronic displays. (Further, although they came into effect in July last year, manufacturers have a 2-year grace period to make spare parts available). So Honda aren't obliged to make a spare part available for your machine.
Your only chance of forcing action would be to use the the Consumer Rights Act 2015 to force the vendor (not the manufacturer) to repair or replace your strimmer. In order to do so though, you'd need to be able to prove that the current problem has arisen because of something which was actually wrong with the strimmer when it was sold to you. (e.g. that a poor quality part was used in its construction). That might not be at all easy to do.
So I think that your legal options are, in reality, going to be non-existent. If you can provide either the actual part number of the handle or the model number of the strimmer, there might be someone here who can help you track down a spare part though.