Just contact some builders and ask them to trot along and explain what they would do to make your house warmer. A good builder will be very well clued up about insulation to the latest standards and new materials are extremely cost-effective.
I would certainly not add insulation to the outside of a house.
Well you’re a fine one for advice. Most builders don’t know the first thing about building science, hence why you’ve probably given such duff advice about external insulation.
As matt said, it’s been far more common across Europe for far longer. The UK is getting to grips with it and it’s becoming far more common and that is a very good thing. You can get far more insulation outside than you can inside, so that’s in your favour. You also have an insulated external layer with a solid thermal mass inside, and that is the optimal place to have any thermal mass in a building.
OP – look round for specialist external insulation contractors in the area and seek recommendations. It’s a very good way of improving the performance of your house.