Of course you can get beer in bottles but those metal caps have a plastic liner or small disc to prevent leakage. It’s a tiny amount but it is plastic.
Cans of beer  are plastic lined!
The only way to get plastic free booze is to use a refill service. Take your own flagon to the alehouse and get them to fill it. These are the ones I know of

Huddersfield

The Sportsman Huddersfield  will sell you your own reusable flagon. Yes its plastic but it will last you a good long time.
The Grove
The Head Of Steam ( also sells plastic flagons)
All sell great real ales and very reasonable priced. 

Magic Rock sell metal reusable flagons. Very stylish! Also sells  great real ales but they are more expensive.

If anyone else knows of other beer refill options please leave a comment below… many thanks.

London

Hi Kate, I have been out and about in Kentish Town again  A review of Clapton Craft – refill beers and wines. https://arefugefordaffodils.wordpress.com/2016/04/14/green-shopping-in-london-clapton-craft-in-kentish-town/

You can find other plastic free beverages here…