> Why would anyone choose subsistence grain agriculture when literally anything else was available.

There was nothing else available. If you were born in an agricultural settlement, that was probably the only lifestyle you knew. Even if you knew other ways to sustain yourself, all land was already taken. Settled regions were too densely populated, and the lands beyond them were claimed by indigenous peoples. If you wanted to migrate there, the best option was to go in sufficient numbers and start a new defensible settlement.

Middle-Eastern farmers started migrating into Europe ~9k years ago, and they reached the British Isles and Scandinavia ~6k years ago. The population that reached the British Isles was ~80% Middle-Eastern and 20% Indigenous European by ancestry.