The pictures you show look like Nuphar lutea, which is common in the United Kingdom and plants can be bought from commercial aquatic garden centres throughout the country. The only Nymphaea that is yellow as far as I know is Nymphaea mexicana. There is a colony in the Oxford University Botanic Gardens I think.
The plant in your pictures are definitely Nuphar lutea, which is called spatterdoc here in Florida where it is considered a native.
Nymphaea Mexicana is the only Nymphaea sp that I know of with yellow flowers, they are fairly pale yellow.
You may be able to order seeds of either species. Be aware however that Nuphar lutea grows very large rhizomes that are usually in the organic matter on the lake bottom.
I agree with the previous authors. Nuphar lutea is a native water plant in Slovakia occurring e.g. in the oxbows of the Danube. It is also cultivated in ornamental garden lakes.