I think the absolute maximum this country can withstand anytime soon is around 3%. Any higher it will cause serious economic hardship for many people, businesses and the government.
Mortgage rates are currently roughly base + 1%. If rates went up to 5%, the effect would be.
£100k - £400 -> £644 per month.
£200k - £800 -> £1,289 per month.
£300k - £1,200 -> £1,933 per month.
£400k - £1,600 -> £2,577 per month.
£500k - £2,000 -> £3,222 per month.
£300k isn't an unusually high mortgage for a recent first time buyer. A rise to 5% would see them loosing £800 a month. The BoE doesn't need to raise rates that much to have a huge impact on peoples budget. Especially when people face high price inflation and little to no wage inflation.
The average UK mortgage is £120k, and in London and the South East (roughly a quarter of the UK population) it's in excess of £200k
The 0.25 rise alone will be enough cause some serious jitters. It will be interesting to see what unfolds over the next couple of months.