Problem will P21s as a standalone product is it will look great, but durability is only a few weeks. Fine if you want to reapply it every few washes.
You could just prep the paint with any polish and paint cleaner and then use some wax- SRP plus P21s would look great just not offer than much durability in the protection.
Spend the extra time doing the EGP and you will get 3 months odd protection.
Pushed for time wash, SRP and EGP the car one day, then add another coat of EGP the next time you wash it, and then a coat of P21s after the next wash to break up the process.
If you would really just rather do a cleanse and wax routine I would look at a more durable wax.
I would probabaly do Swissvax Paint Cleaner and Swissvax Onyx Carnuba Wax to seal.
http://www.cleanyourcar.co.uk/swissol/cleaning/swissvax-cleaner-fluid-250ml/prod_247.html
http://www.cleanyourcar.co.uk/swissol/swissol-wax/swissvax-onyx/prod_230.html
You could also use the Onyx over the SRP and EGP for an improved look than the P21s gives, but it is double the cost to buy.
