The Fluidmaster 747EL 3/8" Side Entry Fill Valve
 with a brass thread is a quiet, durable, and highly adjustable float valve suitable for both high and low-pressure systems in side-entry cisterns. It is widely available from various plumbing and hardware retailers in the UK. 
Key Features
Connection: Features a robust 3/8" BSP heavy-duty brass shank, which is better than plastic threads at preventing cross-threading. This is the smaller, "European" standard thread size.
Performance: Described as extremely quiet in operation and offering a fast refill. It also has an anti-siphon/backflow design to prevent contamination of the fresh water supply.
Compatibility: Compatible with virtually all side-entry cisterns and works efficiently with both high and low water pressure systems.
Maintenance: Designed with a unique snap-release system for easy access and servicing of seals, which are simple to change