Are we heading for a stock market crash? AI-related stocks currently account for around 49% of the S&P 500’s market-cap. And that’s reportedly a higher single-sector concentration since the US railroad boom of the 1880s. The possibility of an AI-led crash is rarely far from the headlines. And if it happens, I can’t see our FTSE 100 escaping unscathed.

But here’s the good news. I see plenty of UK stocks I rate as strongly defensive, and good candidates to consider buying as protection against, or after, a fall. Today, I pick three and suggest a key attraction plus a specific risk for each one…

Essential health needs: GSK

• Attraction: GSK (LSE: GSK) provides a number of essential medicines and its vaccines research is among the world’s best. Whatever the economy’s doing, or wherever stock markets are going, these should provide GSK with a defensive revenue stream. The forecast price-to-earnings (P/E) ratio of 14.5, dropping to 12 for 2027, doesn’t appear stretched.

• Risk: The key near-term danger I see is HIV drug dolutegravir facing loss of US patent protection from 2028. It accounted for about 17% of 2025 sales, and its loss could put a dent in profits. Investors need to keep an eye on the drug pipeline.

Keeping us sustained: Tesco

• Attraction: As the UK’s biggest groceries retailer, Tesco‘s (LSE: TSCO) defensive advantage seems clear. According to Kantar Worldpanel, it currently commands 27.8% of the country’s supermarket share. It’s sheer scale and purchasing power should help it compete even in an economic slowdown or a stock market crash, when shoppers are feeling poorer.

• Risk: Intense competition’s squeezing profit margins. And with a forward P/E of 17.5, there might not be a lot of safety in today’s valuation. It could be a top one to consider buying should prices fall in a slump.

Everything needs energy: National Grid

• Attraction: National Grid (LSE: NG.) has a monopoly over the UK’s energy distribution networks. And that’s supported by long-term energy demand — no matter what the stock market’s doing. Being in a regulated industry provides long-term revenue clarity. That’s helped National Grid lift its annual dividend for 26 straight years, though it was rebased in 2024.

• Risk: The company’s huge investment programme could be a major drag. In the 2026-2027 trading year, capital expenditure is expected to reach £13bn. So costs of servicing debt could put pressure on dividends. A P/E of 14 is about average for the FTSE 100.

What’s the bottom line?

The secret to surviving stock market crashes, surely, isn’t to try to time them — which we really can’t do with anything close to accuracy.

Maybe the next crash will come in November, or maybe not for a number of years. Either way, it’s surely good to keep our Stocks and Shares ISAs diversified, with long-term gains in mind, and encompass a decent proportion of defensive stocks.

I don’t hold any of today’s three because my ISA’s almost entirely defensive anyway. But I rate them as solid contenders to consider for this strategy, along with a number of other UK stocks that currently look great value…

Should you invest £5,000 in GSK right now?

When investing expert Mark Rogers and his team have a stock tip, it can pay to listen. After all, the flagship Twelfth Magpie Share Advisor newsletter he has run for nearly a decade has provided thousands of paying members with top stock recommendations from the UK and US markets.

And right now, Mark thinks there are 6 standout stocks that investors should consider buying. Want to see if GSK made the list?

Alan Oscroft does not hold any positions in the companies mentioned.