A flitch plate is a thin steel plate sandwiched between two pieces of timber. The idea is that the steel plate takes most of the stress, while the timber members each side stop the steel plate from buckling sideways. For most applications, flitch beams went out with the Ark.
The plate they have specified is 20mm thick and 195mm deep, and good luck with drilling that and lifting it into place because that is not 'engineering', it's just nonsense.
Those sheets you have shown are not proper calculations, and if I was building control's checking engineer I would just throw them back.
As it's for a loft conversion, ask the bod who churned out that rubbish to give you a hand lifting the flitch beam into place and see what he thinks then.
For a 5.3m span, a steel beam (if necessary in two lengths to be bolted together) would be far easier and probably a lot cheaper.