I think the likelihood of Britons (or any European) holidaying in Asia this summer is basically zero and next winter is relatively low.
Countries like Thailand, Malaysia and Vietnam will first need to vaccinate a large proportion of their own population and then wait and see how well the vaccine works in terms of stopping the virus spreading and reducing any harmful effects.
When they do open up, it is likely to start with limited and controlled tourism. That means, most likely, tour groups from countries with very low infection rates and strong government controls over the virus.
Dependent on vaccination and virus prevalence rates, Europe is much more likely to open up this summer.