He would genetically engineer cows to produce highly explosive milk. It’s easy enough to do with a basic chemistry/biology lab these days.
Then, very carefully, he would milk the cows and produce several tons of bio-engineered exploding camembert.
This he would leave to ripen until it was good and smelly – the smell would confuse any sniffer dogs or explosives detectors.
Next would come the tricky part. He would make several hundred camembert sandwiches – careful with those slices of bread – and then take them down to the Houses of Parliament. With the help of an unwitting celebrity chef (Jamie Oliver, natch) he would then hand out free sandwiches to passing MPs, dignitaries and even ordinary tourists.
Stomach acid would react with the cheese, resulting in a massive conflagration engulfing everything in a massive explosion of dairy products.
Job. Done.