Lasius flavus - yellow meadow ant
Yellow ants found nesting in large mound nests in grassland or meadows will be Lasius flavus. Yellow ants found elsewhere could be another species of Lasius although chances are high that they too will be L. flavus.
More information about Lasius flavus
Photos: worker Lasius flavus showing its yellow-brown colour (by Josef Dvorak) and a typical scene of L. flavus nests with distinctive mounds in grassland (by Rosemary Winnall)