I think you are in a hurry. Try to take a step back and reconsider the whole situation.
How much did you pay for the car? I had bought mine for £4.5K at 6 years old from the dealer with the history and the timing belt done, at 76K miles. Paying £2.5K to rebuild a fragile engine with 150K miles on it does not sound very wise. The same dealer has a 2008 diesel 6 now for sale for £3K and it is a 64K miles car. If you pay £2,500 for the repair, sell your car for £500, catch the next train and buy the one from the dealer with some warranty.
https://www.arnoldclark.com/used-cars/mazda/6/2-0d-s-5dr/2008/ref/arndl-u-10665A camshaft, plus the seal and the sensor, will not come cheap. You can easily throw £200-£300 here.
Another £200-£300 for the timing chain, water pump, auxiliary belt.
Another £50-£100 for the lifters and anything minor extra to close it up.
Plus the £50 to the guy to confirm what is going on.
Plus the injector seals, plus transport, plus, plus, plus.
You have paid easily £500 before you start rebuilding it. I think you have to be realistic and expect that will be closer to £1,000 as a final cost to DIY the work. But if you end up transporting the car to the garage, you are already almost fixing it there. So you will end up with £2,500 gone.
Price up the parts that you already know that you are going to buy and all other costs like the garage, the transport, etc, then call the scrappers and find out how much they will pay for it. Similar cars are on sale on Auto Trader from £500 for non working cars like yours.
My guess is that your particular engine will not last very long, even after a very careful £2,500 rebuilt by a professional. There are so many GH cars which had multiple chains one after the other, without breaking up like yours. Your engine will have tolerances far beyond the original engine. So I think that 1-2 years later you will be back on square one.
If you can get somebody to come over to your home and check the car, give that £60 or whatever they ask, to keep your mind in peace that you tried.
https://www.whocanfixmycar.com/As I said before, they may come around to have a look for free.
If they give quotes in thousands like the £2,500 quote you already have over the phone, think very seriously whether this is the right way forward. The 2.0 belt engine will keep going for ever compared to what is going with the 2.2 chain version of the same engine.
If I was you, I would advertise it on Auto Trader for £500. I expect that you can get £250 for scrap value. If you manage to sell for £500, you save yourself the trouble of having to break and sell the car bit by bit.
I would use the £2,500 for the repair to buy another car from a franchise dealer. If you want to buy another 6, you can buy another for not much more than the £2,500 repair bill as you can see from the ad above. I would not trust that entering the garage for £2,500 you will not end up driving out with a higher bill.
If you end up buying another 6, you can keep yours for parts.
If you decide to buy another 6 from an independent dealer, keep in mind that most of these cars are part exchanges which cannot be sold at the franchise dealers because of the faults they come with. Faults include the regular maintenance needed to keep it going and which costs a bit too much for the dealer to guarantee and sell them. So they sell them at the auctions for £500, plus auction fees, plus transport, the £500 becomes £1,000, and then the independent dealers double that price and wait for the sale. There is a chance that bailing out of a £2,500 and buying a £2,000 independent dealer car from the auction, may bring back at the same position and the extra £1,000 to buy a franchise car is better value for money.
Anyway, I do not want to destroy the spirit. I can see that you are happy for DIY and by all means give it a go. Even if you pay £500 for parts and then sell the whole car for similar money, you lose very little. But what I would not encourage is a £2,500 engine repair even from an engine specialist. It is just a bit too much to risk. Sleep on that and gather more opinions and information before you commit in anything.