Am I missing something here
Exactly. Assuming having an extra pint of milk in his 2l bottle at home won't cause the milk to go sour before it's all used, there is very little advantage to buying a special milk container.
The difference between a pint bought singularly and a pint bought as part of a large pack is perhaps 2 or 3 pence. Even assuming the guy works 50 weeks of the year and the difference is as large as 5 pence, he'll only have cost himself £1.50 over a year by just buying a pint bottle.
Any bottle with a reasonable chance of keeping the milk in will cost around a fiver, and the OP gives himself the job of decanting milk and washing the bottle regularly. And he has to buy a special STW sanctioned boutique bottle brush, to go with said bottle.
Can we have the OP sectioned?