The scale of it is genuinely hard to pin down because so much slips through undetected. That £1.3bn figure you mentioned is plausible - various industry bodies and police reports do cite numbers in that ballpark - but the actual total is probably higher since not every fraudster gets caught and some claims go unexamined. The insurance industry itself does investigations and publishes data, but they're naturally focused on their own losses, so you're not getting a complete picture of what's happening across all sectors.
What makes this tricky is that "fraud" covers everything from outright fabrication (like your Lego example - someone claiming expensive sets were stolen when they weren't) to exaggeration (padding damage claims) to opportunistic lies on routine forms. A lot of cases never make it to court or even formal investigation, so they disappear into insurance company write-offs. You'll see higher estimates if you include things like organized crime syndicates targeting businesses versus petty individual claims.
One practical angle most people don't think about: if you're buying anything expensive, photograph and document it before anything happens. Serial numbers, purchase receipts, condition shots - all of it. Insurance investigators actually respect this because it makes fraud harder to pull off convincingly, and legitimate claimants with good documentation tend to get processed faster and without as much hassle. It's not foolproof, but it shifts the odds against the people trying to game the system while protecting you if you ever need to make a genuine claim.