Mine cost £150 but that was more than 20 years ago - it's a 1/5 carat diamond solitaire in white gold (not unlike the OP's but mine is in a twist setting). You wouldn't get a similar ring new for that price nowadays.
An engagement ring is - or should be bought with the intention of being - a once in a lifetime purchase that you can see yourself wearing for the rest of your life.
I really don't think £1200 is an unreasonable amount to spend, or an unreasonable price to mention. There used to be a 'rule' that an engagement ring should cost about one month's salary - well, if OP's fiancé were on the minimum wage, £1200 would be less than half that amount!
The OP mentioned the price because some people were criticising the solitaire for being too small - in other words, hinting that her fiancé had skimped on the cost.
For all you know, he might have been saving for months to buy the OP a nice ring.
I do think you are being a little unreasonable,