So once you have completed this job in a couple of weeks (83 hours) at your new quoted labour rate of £2490 (plus VAT if registered) will you hang around to deal with any post installation issues the customer may have or be back to Glasgow? What warranty will you offer on your installation work (I give five years) and what arrangements will you make to service this warranty? It's a long way to travel to Kent for the sake of a leaking TRV.
All of the above assumes you can actually complete the job in the period allowed. Don't forget that in addition to the central heating installation (boiler and cylinder in loft which will slow you down), there is a new gas pipe to run, new soil stack to install, bathroom, ensuite and cloakroom to install and last but not least a new mains water pipe to run from the pavement. Whilst in theory it could be done, completion in your timescale will depend on other trades being on track. What if the tiling isn't completed in the bathroom and or ensuite; walls aren't plastered/decorated ready for radiators to hang; electrician hasn't got round to supplying power in loft; bath ordered by customer is delayed/damaged. Is the house going to be occupied during works and if so are you having to move furniture, lift carpets etc., or will you arrive one day to find that other trades have nicked all the parking spaces and you are relegated to the adjoining street where some b*****d nicks all your tools.
Assume you have sorted all the above, you're going to need somewhere to sleep/eat. You look for a local B&B only to find they're all full with the latest batch of immigrants shipped in from Dover. The only ones with spaces want £50 a night and that doesn't include lunch or dinner. Add fuel for the return trip and you could easily be looking at another £1,000 - £1,500 in expenses.
I'm genuinely sorry to hear that works hard to find in Glasgow at the moment, however there are no rich pickings to be had in the South East (those days are long gone). If it was that easy, you can bet the lads from Birmingham/Manchester or the Eastern Europeans would be there before you in any case.
If you do come down, I will at least guarantee to buy you a pint of our finest.