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The regulations are explained in the UK Border Force leaflets Bringing Food Products into the UK and Bringing Fruit, Vegetables and Plant Products into the UK, and the online guidance on personal food, plant and animal product imports. The general rules on bringing goods back to the UK also apply when those goods are food: you'll owe customs duty and possibly also import VAT if you bring back goods worth more than £390 per person and excise duty on alcohol and tobacco beyond your personal allowance. All of the following assumes that you're not bringing back any endangered species.
The relevant parts are:
You can't bring any dairy products into the UK, except from the EU, the Faroe Islands, Greenland and Iceland (food leaflet, p. 4).
You may bring any quantity of chillis as long as they're for personal use. You don't need to get them inspected in Mexico and you don't need to declare them to UK customs. (Fruit and veg leaflet, pp. 7–8, 13.) Different restrictions apply to other kinds of fruit and vegetables; see the leaflet for details. Also note that the government's web page about bringing food, animals or plants into the UK says you're only allowed 2kg total of fruit and veg and that it must be "free from signs of pests and diseases."
You may each bring up to 2kg of "other animal products", which includes insect meat (see the guidance). I'm not sure if you have to declare these to customs or not; best to err on the side of caution unless you can find an explicit statement.
As long as you're travelling on the same flight and pass through customs together, you can combine your allowances so, for example, you wouldn't have to split 4kg of insect meat into a 2kg package in your bag and another in your wife's bag (see the section "Seizures" in the guidance).
If you bring something back and you're not sure if it's allowed, declare it to customs. If it's allowed, they'll just send you on your way; if it's not allowed, they'll confiscate and destroy it, but they won't take any further action against you (see the leaflets). If, on the other hand, you don't declare something and they find it, you're smuggling and you could be prosecuted.