Fairly clear to me - and if you think of it, duty free is for export out of the duty country/region.
Most UK airports have a decent choice. If flying with Ryanair and don't have bags booked, it has to fit in the small carry on bag - but if you have booked bags, then it can be carried separately as a third bag.
Q. I am an Irish resident flying to the UK and I want to buy Duty-Free on my way out at Dublin/Cork Airport. Can I use Shop & Collect?
A. From 1st January 2021, all passengers flying to the UK can avail of our complimentary Shop & Collect service on all products, excluding alcohol and tobacco. If your basket includes mixed products, for example, alcohol and beauty products, your basket may be separated and you can avail of Shop & Collect on qualifying products, as outlined above.