I'd suggest the issue is probably with the new pads.
The discs make no difference at all. They are hunks of cast iron, that's it. The pads are the vital factor. If the garage called a local factor for pads, they will use the cheapest - £15 a set is perfectly normal.
Good quality pads by brand names start at about £30 and can go over £100 a set.
Garages fit the cheapest to keep profit margins high.
I'd suggest you go back to the garage as ask to demonstrate the problem.
However, they will probably point the finger at
Servo, brake lines, fluid.
The fluid should be changed every two years. If not done, budget £30-£40 for a garage to do it.
Servo assistance is an easy check. With the engine off. Pump the brake pedal 5 or 6 times until it's hard.
Keep your foot on the brake and turn engine on. The pedal should move downwards towards the floor. You'll feel it. If it does the servo is fine.
The brake lines are harder. They often perish inside and you can't check then as they look fine outside. Only fix is replacement. Again, cheap. £8 each or so. Garage will charge £30-£40 on top to fit.
If changing them, do all 4. The fluid needs changing with this job so get a price for new flexible brake lines and fluid together. It'll be cheaper that way.