Personally for me - I commuted on an alu bike during the weekdays then carbon bike during the weekends for various reasons.
1. Different bikes, different handling, different rides
2. Roads in the city are slightly quieter during weekends. (London city roads are quite busy)
3. Alloy is more durable than carbon should i end up getting hit by something - One less thing to cry about and expensive thing to replace.
4. (personal opinion) I dont take my carbon bike out in cold, wet or damp weathers. You dont want your best bike out in the worst weather.
YMMV
I would however suggest maybe spending the extra money and getting a titanium 'do-it-all' bike if your budget can afford they are as light as carbon (if not lighter) are super strong, quite weather resistant and pretty comfortable to ride.
though since youre just starting of maybe a titanium might not be the best suggestion but they should hold their resale value pretty well
I would start off with an Alu frame like a £600-800 Triban then spend £200-300 on getting some hand built wheels to carry your weight. I weighed 16stone when i started cycling and had both my wheelsets built by
@Spoked Wheels but I think he has since stepped back from wheel building.
A good local bike shop worth their salt may build you a set if you ask them to but if not there are websites that make strong wheelsets for the reverse backpack carrying cyclists
https://www.stradawheels.co.uk/product/big-fella/