...of course you could pay it and then bubble him/her to HMRC for non-payment of tax on earned income (once you've safely got your CPL, of course!)
Examiners undoubtedly need to earn money to live, just like the rest of us. I recall my 170A test - the fee was included in the aircraft hire at the CPL training rate.
Put it in the context of having building work done and paying a lesser amount for cash, i.e. the builder doesn't declare it for income tax or VAT. I guess it all depends upon your moral stance on the 'Black Economy'.
The 'declared' rate would work out at around £150, if the tax evading rate is £100.
As you correctly say, paying cash leads to all sorts of moral issues around bribery and corruption. A slippery slope. It would be a pity if we descended into this and became like some other nations!
TheOddOne