All of my councils are responsible for their street lights. These assets should be covered by an inspection and maintenance contract with a contractor suitably qualified to work on the public power network, with annual safety inspections. You should also have suitable emergency call-outs as part of the contract, for example if a car hits a column at 2.00 on a Sunday morning and it needs to be made safe. The contractor should also be able to provide you with an estimated remaining lifetime for each light. Finally, you should ensure that your insurance company is aware of these lights and that they are fully covered in your public liability insurance package.
You may be able to access maintenance cover at a more affordable rate through your district or county council, or through a shared arrangement with neighbouring parishes. On a large contract, you'd expect to pay around £50 per light per year for cleaning, inspection and testing to comply with the legal requirements, but individually I suspect you'd pay a lot more.