You're not an idiot. With the plethora of bottles of stuff out there it's hard for many people to know what they need to use. You need something with NPK and calcium, magnesium, sulphur, and other micronutrients. None of which are being supplied by Big Bud except for some P&K.
For a one part dry nute that provides all that MaxiBloom would suffice. Or a three part nutrient like GH Floranova that has grow, micro, and bloom parts. Basically anything that has micronutrients and still provides some nitrogen during flower. I'm not in the UK so I don't know what is available.
It would be easier for you to post what you can get or maybe someone in the UK can chime in and assist because we have access to stuff in the US that you don't. Maybe House and Garden A & B, or Canna, Advanced Nutrients Sensi Bloom A & B. There are too many to name. But you need a complete nutrient.
Good luck.