There are basically 2 types of questions discussed in this thread so far, so I will comment on both of them.
1. How to present yourself as trustworthy. Honestly, you don't need much. Usually, it is very often to spot a scammer here. They always ask for some more valuable coins or banknotes and are very quick to jump into agreeing, even though it seems not to be fair from their side. But there are not that many scammers here, fortunately. So, just start negotiating as a real swap you want to complete, and agree to send first. Sometimes if the swap is not that valuable, people will be ok sending the coins at the same time, even though you are a newbie, but still propose to send first. And just do a regular negotiation/discussion, without being unfair. If you are sending coins first, there is almost no risk for the other swapper, he will just wait for your coins to arrive and then send his/hers. So, don't worry about it.
2. The more complicated thing is actually to agree on the swap. Because you need to have some coins that the other person needs as well. And it needs to make sense to ship them, especially abroad. As Sjoelund mentioned, you should add more coins to your swap list. It may seem that your British circulating coins are not of interest to anyone, but they are, especially for those who live abroad. So, just add a bunch of British commemorative circulating coins to your swap list, especially of the very recent years because those will be that people miss in their collections. Add even a few 2022-2024 non-commemorative regular coins, some people would even want those.
Start swapping with people that you have a low postage cost first to build your swap ratings.
I also commented at some point earlier here about swapping, maybe read this thread, I am sure it will be useful.
Cheers!