I think the coils are £30 maybe closer to £40 each (unless its a DIY job, best replace the back 3). You will need to spend £50 on inlet manifold gaskets. Plus it would make sense to get new spark plugs, so an extra £24. Its about 1 hour and 30 minutes in labour. I would not pay a garage no more than £300 for the job including parts. You could source the parts yourself, maybe get some 2nd hand coils from a low mileage car.
https://www.autoreservejaguar.com/v6-ignition-coil-pack.html
You could probably get labour down to £150 if you find your own parts.