There are a thousand different routes to a modular frozen ATPL:
The most expensive is to do a CPL in a Multi then a full MEIR course.
The cheaper is IR(R) - MEP (course only) - CBIR (multi) - CPL.
There’s no getting away from the fact that you have to pay to fly a plane and build hours. So it’s always cheaper to get the IR first, and take the CPL at 200 hours. Combined with hour building, an IR(R) costs about £1000 for FI and Examiner etc. Building Instrument PIC time is then free and the CBIR requires 10 hours of multi engine time.
You can get an IR for £6,000. I’m surprised anyone still does the full course.