You've the guts of £45-50 for materials for a CO detector, so £20-25 for labour, can you fit every single one in under an hour? Possibly going back to CU, and a new MCB. I'd be looking your price for replacement never mind installation.
The PAT testing at £50 may seem low until you have an hour or more drive each way and are there for an hour or 2.
Basically you need to work out what hourly rate you need to be charging, then estimate how many man hours for each task, add in materials and you'll get a more accurate price that you should be charging, then give that as the minimum or approximation.
Be realistic in estimations of work supply, if you pay yourself for a 40hr week at say £600, x52, then add in all your other expenses, insurance, any other costs like tools and vehicles etc, then divide that by 46, as you should be allowing for 6 weeks of no earnings to cover your own holidays, Christmas, Easter etc etc.
Now you need to estimate how many billable hours you can charge each week, remembering that you will have travel time between jobs, so for calculations we go with 30 billable hours, you can deduct a bit if you are all day or multiple days at one job, as you will be able to bill more hrs and have less travel costs. To stay competitive on larger jobs. If you know your weekly target then you can calculate a day rate for the bigger jobs.
By my calculations it's around £27-30 minimum to be able to get the same wage as if you were on the cards for someone else. So you either increase your labour rate to reflect the extra stress and admin work, or you add up to 30% on materials. To me the extra on materials is best as some folks tend to assume that your labour rate is actually your hourly wage, forgetting that it's covering a whole lot more.
Now these are rough estimates, only you know your expenses and desired target wage, so work that out and start from there.
