Hi OP,
I think you are very wise to tackle this now before your fixed rate ends and the shit really hits the fan.
I think some people replying are underestimating the scale of the problem. You already feel like you are not able to take part in normal little treats like cinema or a meal out and lack the ability to deal with a washing machine breaking or a car repair without using savings.
£230000 at 2.5% over 30 years is £909 a month - I think that is broadly reflective of your mortgage.
By 2026 if your mortgage amount owing is down to 220k
£220000 at 6% over 27 years is £1,373 - so almost an extra £500 a month - with no guarantee it won't be higher than this.
I also think the suggestion your current house will hold it's value well is naive. House price falls are predicted. I've never heard so many people in real life voice out loud that they are glad they didn't upside as they wouldn't have been able to manage, read so many stories in the news about people who can't afford their mortgages, or seen so many threads on mumsnet about people needing to downsize.
There are many people in small houses that can't manage too but they will cling on for dear life as their alternative is an overpriced and insecure rental property - if they can get a property at all.
OP - downsize in a heartbeat. Be mortgage free or very low mortgage. If you and your child lived in a comfortable two bed now with comfortable finances you would never in a million years be looking to move to a four bed detached which stretched your finances. The absolute last reason you should be staying is because of what houses other people live in or what other people think. How much thought will those other people give to you when you are running up debt and ccan't afford to send your child on the school residential? It is you and you alone responsible for your child's quality of life and they will never grieve not having grown up in a 4 bed detached. They may grieve however a stressed/depressed mum, and being excluded from an ordinary quality of life and activities.
You know what you need to do.