This might be of use to some light users. ID mobile are currently offering the 4G ready Alcatel Pixi 4 5 Inch edition for £6 per month on a 2 year contract. £144.
Possible £21 Top Cash Back also available. Taking the total to £123 or £5.13 per month.
This is the cheapest handset and contract that I have found for a half decent 4G phone running Marshmallow .
Contract details :
1.0GHz quad-core processor
Android 6.0 Marshmallow
8GB expandable to 32GB