1.) It's not good for your health. Caffeine isn't especially great for you and even if you're not buying a coffee, but a hot chocolate instead, there is a layer of plastic lining the inside of those "paper" cups. When they jet spray that scalding boiling water into that cup, it could melt and seep into your drink. Rub your finger along the inside of the cup next time you drink from them. I swear you can feel that plastic film come off onto your fingers.
2.) It's bad for the environment - Because of that layer of plastic on the inside of the cup, they cannot be recycled. You'll enjoy that coffee for a max of 30 minutes, and then you'll throw that cup away, and that plastic lining will linger on earth for hundreds of years, polluting our earth. Just make your coffee at your workplace in a nice mug!
3.) It's a waste of money - most work places provide a kitchen, just make your coffee at work, why are you wasting £3 on a coffee it would take you 5 minutes to make yourself. Three pounds a day that's £15 a week. That's £15 to drink coffee which might contain harmful plastics, and pollute the environment. That's £780 a year! That's £3.9k in 5 years! I could go on.....
4.) Tax avoidance - Even if you thought that £3 goes towards paying someone's wages and someone's taxes. Think again. These corporations tend to be massive tax avoidance companies. Recently they have bowed to public pressure to start paying more taxes, however they still have very complex tax structures in place that are totally not transparent enough to know if they are genuinely paying their fair contribution to society. If that local coffee shop if paying 20% tax to the UK, so should these multimillion pound companies.