Sometimes they cost absolutely nothing. If you wish I can send you one for price of shipping. Look in local classifieds for bargains.
You generally get what you pay for.
60 cm steel dishes can be real cheap, you can probably order one for, say, 25 GBP + shipping from e.g. Germany.
Performance and longevity may vary.
I buy Telesystem 80cm dishes really cheap, but eg Triax cost more, Gibertini even more.
If you want aluminum (no rust) it's slightly more again, but still manageable.
But be aware that the Sky Q LNBs have a custom feedhorn with an illumination pattern that matches the Sky supplied dishes (oval), so your round "normal" dish may not make a huge difference in reception.
When the Sky installer does the install, Sky warrants reception. This is why they won't let you use your own dish. Too much variation possible.
But, a question begs: Why do you need the bigger dish?
Astra 2 at 28.35 +/- 1.5 is strong in most of UK, so a standard Sky dish will work well (in the average life-span of your Sky subscription).
IMHO: If you want to experiment with multi-LNB configuration on the dish etc, this is possible with mixing the Sky LNB with other LNBs on a normal dish, but it is generally much easire to leave the Sky dish for what it is, and then run everything else off the other dish.
Disclaimer: I don't currently live in the UK, nor am I a professional Sky installer. Therefore there may be other, more knowledgeable member of this forum who knows more detail about the situation. Please chip in if you want.