The first three. On the current tariff rules you can choose whichever you want, and change it if required, which is something I found useful to do, on my older tariff which was similar to yours, but lower, I had no such choice, it was 500mb for £5 like it or lump it.
On the older tariff the bolt on was £5 regardless of the amount of data you had, and the level of data you had was determined by the price of the tariff itself, which was unfair to anyone who used few minutes or texts but wanted a decent data allowance as you had to pay way over the odds to get it. It's now possible o get 1GB with 100 minutes or even only pay for 100mb on a 900 min tariff to make it cheaper if you don't use a lot of data, for example. I realise it doesn't look so good from your perspective of having 750mb for £5, but i think having a choice is a good thing. I was saying you were wrong only in the sense that you stated the 500mb bolt on was the only one available to you.
While in your case you do get less data for £6, this can be said to be due to the general unfairness of those older tariffs as all customers on 600 mins or less got 500mb for £5 anyway (which you would have dropped to without saving any money if dropping your tariff under your current t&c's) and with the new one you do get mms and tethering included and surely a standardised price for everyone is fairer?
