thank you, the AA guys said the battery was fine and told me it was the alternator which is why I brought this up. If the alternator is dead ( because the voltage did not rise above 12 when the engine was started, will this have an effect on the new battery?
Ok. You jump started the car and then checked the voltage with the jump battery still connected? If the alternator was working you should have seen at least 13 volts - more likely 13.5 to 14 volts - even with the engine running at idle ad certainly even if the revs were raised to as little as 2,000 rpm. The fact this didn't happen points to either the alternator being clapped out or a problem with the wiring. Wiring can differ but generally there will be a thicker wire which connects to the battery so should test out at battery voltage all the time and a thinner one which will only show battery voltage when the ignition switch is on - this smaller wire provides electricity to the field windings and varying this supply, via an ECU, is how output is controlled. As the AA guy diagnosed a "dead" alternator I think you can probably take him at his word. It's not a difficult thing to diagnose if you know what you're doing.
You say at one point you checked the battery and it read just 2.6 volts? If your existing battery is a stop start (most likely will have EFB or AGM on it somewhere) then letting it drop to this level will have almost certainly "killed" it. Old "wet" lead acid batteries were more tolerant of this sort of treatment but more modern batteries, the types mentioned above, really don't like it and "sulk" by going into a "deeply discharged" state from which it's very difficult to reawaken them. Regardless of the alternator performance, you almost certainly need a new battery.
Replacing both these components should sort the immediate problem but, if it were mine, I'd be checking for parasitic drain just in case something is robbing electricity while the car is standing. I do this by disconnecting the battery negative terminal and connecting in an ammeter between the terminal on the battery and the earth lead. I have a large current ammeter - 0 to 10 amps - with a 15 amp fuse in the lead just in case there's a dead short somewhere. Testing with an ammeter is potentially dangerous as an ammeter is basically introducing a dead short - ie, nil resistance - connection, so the potential to "blow things up" is great. If the 10 amp meter doesn't flicker then I stick my multimeter on in it's place and wouldn't expect to see more than 100 milliamps. If you're going to do this you need to open the bonnet, make sure everything is switched off, close the doors and give it a few minutes for all systems to power down. If your car has one of these infotainment screens then it may take a few minutes for everything to go dormant. I like to give it a quarter of an hour before doing a parasitic draw test.
This might interest you:
https://www.continentalbattery.com/...mal amount of,quicker and shortening its life.
Fitting an old "wet" type battery to a stop start car is not recommended as it won't cope with the constant discharge/charge cycling and will almost certainly fail before it would in an older car without stop start. I suppose that if you could remember to always disable stop start when you start the car then maybe it would be ok as it's the repeated heavy current drain followed by recharging that does them in. It's very tempting because the old type batteries are quite a lot cheaper.
@John202020 above mentioned Tayna, an excellent supplier of batteries as many on the forum will agree. Put your cars reg no into their product finder and you'll see what's recommended. If you know someone with a Halfords trade card that's another place to buy a good (Yuasa) battery at a good price, but only with the trade card.