I think the originator misunderstands airspace nomenclature.. "Airways" is much the same as "TMA" because the TMA airspace is a large piece of Class A airspace, often at the junction of many airways in the vicinity of a major airfield. The rules are the same but the TMA is much busier.
As for filing 6000 ft MID-BIG-LAM.. it would almost certainly be impossible due to confliction with so many routes in the area. ATC would not find much fun in a 100kt Cherokee doing 100kts in the middle of the London TMA - I know; I've seen it and it is not a pleasant sight!
I think your pilot friend needs to brief himself thoroughly before even thinking of doing what you propose. For example, why go at 6000 ft (which is the SID outbound altitude for Heathrow traffic routeing south)? If you are in a light aircraft flying Southampton-Stapleford you'd be far better at 1500 ft SVFR through the London Zone working Heathrow SVR then Thames Radar.