Be resonable !
The thing we have to think about is return on investment, if an airline EMPLOYS you and pays for your type rating they should be able to ensure that they get some return on that investment.
A training bond is an acceptable way of ensuring return on investment IF the bond reflects the true cost of the training and the cost diminishes at a monthly rate over say two years.
The problem we have is that pilots are paying up front for ratings and then being taken on as CONTRACTORS, as a contractor under UK tax law you are required to have more than one sorce of income and be able to determin when you do the work. Now what would be the attitude of Easy Jet (lets say) if a contractor said sorry I am working for another airline today? and did not turn up!
It would seem to me that the best way to shut down the pay to fly thing would be to set the UK tax people off in the direction of these contractors, It is likely that if they find the contractors to be Employees then the airline will have to pick up the tax & IN bill!