Is an insulated garage door actually worth it in Manchester, or is it a sales line?

Here it is arithmetic, not sales. Where rooms sit over the garage, an insulated door is part of the floor's thermal system and shows up in those rooms' comfort and in heating fuel. Where plumbing runs in the shared wall, insulation is freeze protection for pipes that burst expensively. Where the garage is detached and unheated with nothing above it, the honest answer reverses: insulation buys little, and the money is better spent on the seal, the hardware grade, and the opener's battery. A Manchester quote should ask which garage you have before recommending a rating, and the recommendation should change with the answer.