Triple glazing gives the best energy balance, even in the UK climate, and is needed to meet comfort requirements and to avoid condensation and mould at the edges.
The glazing g-value (Solar Factor) refers to the fraction of solar heat that transmits through the glass and is just as important as the glazing U-value in this optimisation process, so ask for both to two decimal places. Note that Building Regulations only considers losses, so a manufacturer’s ‘high performance’ glazing will often have a low U-value but also very low g-value which is likely to give a poor energy balance in PHPP. Manufacturers will have a range of glass they can supply, with varying costs so ask for alternatives.
Take care to look at the light transmittance of the glass as well; usually the light transmittance percentage is roughly in line with the g-value.
Other standards such as Secured by Design or Building Regulations, may dictate that security glass is used. Generally toughened glass does not impact on performance but laminated glass almost certainly will.