Seems like you've come across some especially awkward staff.
Personally I've never had any problems buying any quantity I've fancied. including the 59 I bought yesterday (40+19 in 2 transactions).
With Tesco though you should do what others have said, and aim for a small amount per transaction. I recently did 68 in Tesco, but had to put them through in groups of 6 or 10. Took a while but worth it.

It's entirely up to you if you want to go for it - it all depends on how much you value the free Credit Card points and/or clubcard points.
How soon does the tax bill need to be paid?
And do you have the time to bleep in, copy and paste, and submit 120 lots of details from the cards.

The only thing I will say, is make sure you get one of these , they make the job much much easier