I'm sorry but I have to beg to differ... when I worked for Luton & District just after the break up of UCOC in the mid 80's Luton, Aylesbury and Hitchin depots were all on different rates to one another for "historical" reasons {the union's assertion!} and furthermore Bedford depot was on yet another pay rate. The simple truth is that UCOC never had a unified pay scale.
That's a bit of an oversimplification of the agreement. Whilst there were no routes where both companies operated an equal share, there were routes where one company would run the majority, but the other company would have odd workings for "balancing" of the agreement.
Why wouldn't joint operation work? there are many instances right across the UK where 2 companies {or 2 depots} jointly operated routes with staff on different pay and conditions. Even from Luton there was the 142-4 group... majority Bedford but supported by Luton... the 52 group {later to become 92-99} jointly operated by Luton and Hitchin or the 61 Luton supported by Aylesbury.
Of course to say that LC staff working jointly operated services with other NBC companies wouldn't have worked due to union resistance ignores the jointly operated services with Thames Valley, Oxford- South Midland and others.
A lot of water had passed under the bridge between the LT/LCBS split in 1970 and you working in Luton 15 years later! Luton Depot got a higher rate as a direct result of the Corporation takeover [presumably the 'historical' reasons you mention] and Bedford as the result of a prolonged strike and productivity agreement round about 1982. All sorts of other issues had come and gone too.
I never said that Joint operation would not work - UCOC had plenty of instances (131 Bedford-Oxford, 121 Bletchley-Oxford, 428 Bedford-Cambridge, 266 Kettering-Peterborough, 171 Huntingdon-Ely for instance), but these were all NCOI [National Council for the Omnibus Industry] companies and paid the same or similar pay rates. It is easy to forget (or never have known) the nationwide pay agreements prior to the Thatcher/deregulation/privatisation era. Yes there may be some fairly minor differences in conditions, and in some cases pay rates due to historical reasons, but by and large they were the same. As you point out, UCOC had plenty of examples of routes jointly worked by more than one depot (which may or may not have had minor pay & condition differences), but the staff would have grown up with these minor differences and fully understand the reasons for them. It was also very rare at that time for any allocation fluctuation beyond odd journeys on a given route group.
Contrast this with the situation of the LT country area if it had not been split to LCBS, but had been broken up and allocated to surrounding NBC concerns in 1970. Suddenly UCOC would have in its midst a fairly large operation with staff on far superior pay and conditions. This would have been untenable on the Industrial Relations front. It is unsurprising that the NBC did not do this. If some joint operation was contemplated (i.e. work previously run by staff on LT pay and conditions being transferred to staff on UCOC conditions - say the 301 or 321), this just would not have happened. The Trade Union at the time would not have permitted it.
I know that later on the 360 did pass from LCBS to UCOC, but this was (a) a takeover, not joint operation, (b) after most of the staff on LT conditions (as opposed to new staff taken on LCBS conditions) had left or transferred elsewhere and (c) NCOI rates had become a lot better , and a pension scheme introduced in 1973. Ironically the 337/364 services, withdrawn by LCBS in 1971 did (indirectly) come to UCOC, no doubt at a higher pay rate than enjoyed by Court Line/Jeyson drivers. In 1979? a 'joint' service (no. 44) was introduced between Luton and Stevenage via Kimpton 3 days per week, with one bus of each company. However this was new work, and (b) and (c) above applied.
London Transport did have a few examples of joint working (in Slough Route 407 and in High Wycombe Route 326) but these were quite long standing, and had separate route numbers. In the later 70s there were co-ordinated timetables with Oxford/South Midland, not quite 'joint' working, but by then pay and conditions had grown much closer anyway. The fact that there were isolated pockets of certain types of operation should not be taken as an implication that universal application would be possible. UCOC had its own 'joint' route with LT (359) which they withdrew from in 1964 - I think you may find that the pay issue was a factor there.
Notwithstanding the pay issue, United Counties at Luton in the 70s was in no position to take on any part of London Country. After the Corporation takeover buses had to be parked in the streets surrounding Castle Street Depot due to a shortage of space. The Corporation buses were in a poor mechanical condition, and no Corporation engineering staff transferred to UCOC at that time. The Eastern Area Docking shop was at Bedford, and the workload nearly took down the whole company. This was aside of the serious staff shortage at Luton, resulting in a peak of 20% lost mileage at one period. I know LCBS had similar problems on both fronts, but I bet in 1970 UCOC management didn't want a fleet of elderly alien vehicles, more driver and conductor shifts and Industrial Relations problems to add to their woes. Same could be said about Thames Valley I should think.