Potentially relevant standards

EN 15154-2 (EU) Covers plumbed in eye showers
EN 15154-3 (EU)  Will cover tank showers 
EN 15154-4 (EU) Will cover non plumbed eye showers
DIN 12899-3 (German) Covers all body and eye showers for process and logistics
ANSI Z358.1-2004 (American) Covers all body and eye showers

The UK market
Currently there is no overall EU or UK standard that covers the design and instalation of all types of shower and eye bath. Plumbed in eye showers are covered under EN1514-2 and EN 15154-3 and EN 15154-4 cover non-plumbed body and eye showers respectively but there is no recognised EU standard for plumbed in body showers.

The lack of a standard does not, of course, mean that any design of shower would meet health and safety legislation. In the UK there is still a legal requirement to provide sufficient first aid equipment. The lack of a clear EU or UK standard in no way lessens that obligation. 

In order to meet ones obligations as an employer then it may be sensible to look at standards from other countries, in particular the German DIN 12899-3 standard and the American ANSI Z358/1-2004/2009 standard. These both give details on minimum flow rates, shower spray pattern and other design features. The ANSI standard also give some guidance on meeting the obligations to ensure the showers are accessible, visible and not impeded by environmental factors such as frost or heat. Further guidance is given on the quality and temperature of the water used in such showers.