Britain can purchase Australian beef, lamb, and dairy products iron ore, gold, coal, meat, wool, alumina, wheat, which it did before the EU. Likewise it can do the same with NZ. It may also purchase some machinery and transport equipment though there is a lot of competition from other countries.
Britain could import some goods from India and export illegal Indians back to India.
Figures from the UK Office for National Statistics show that in 2015 Australia was #19 in the UK's top 50 export markets (beating out several EU nations, including Poland, Norway, Denmark, the Czech Republic, Austria, Finland, Hungary, Portugal, Romania, and Greece). New Zealand came in at #45. The two nations were #37 and #48, respectively, in the list of top 50 import sources.
Y'all are gonna have to sell the Aussies and Kiwis a lot of Twinings to make up for decreased sales to your #2, #3, #4, #5, #7, #8, #9, and #16 export markets, all of which are in the EU.