Hello! I am in a similar position to you, I have completed my PPL and have now moved to the ATPL distance learning phase. I have little experience with your cost estimations for the CPL/ME/IR, as I am yet to consider myself what to do once I have finished the ATPLs and gained the required number of hours. The CAA exam fees mount to approx £800 on their own, and as far as I am aware you need to be part of an approved modular distance learning course (eg Bristol, ProPilot, CATS, Oxford etc), which are in the region of £999-£2500 for notes and classroom based sessions. All the best - by your post you clearly have a well thought out plan and are entering the industry with your eyes open!
With regards to Tayside at Dundee, I did my PPL in Sherburn, and now living back home, I drive 60 miles in order to fly there - they are a great setup and the airfield has an appropriately good mix of commercial/GA aircraft in uncontrolled airspace to be busy but not as busy as Glasgow/Edinburgh/Aberdeen. As far as I am aware Loganair are closely affiliated with the instructors at Tayside, a lot of them head off to F/O positions - lucky guys!
Also - I notice that you haven't factored in the costs for an MCC or for additional CAA fees (of which there are many), those expensive ones being the class 1 medical, licence issuance, exam fees, etc. etc.