Just click the circle icon in the lower right corner of a text field. The popup that appears has a list of languages where you can select English (UK).

Please contact us at https://languagetool.org/legal/ - this is a bug and we will fix it, but for that, we need more detailed information.

Thanks for your feedback. Could you contact us directly (https://languagetool.org/legal/) so we can work on the performance issue? Many performance bugs have been fixed already, so we need specific examples of where it's still slow.