Can I return to the UK for 6 months while on a Non-O visa in Thailand?

I am doing exactly this since 17 years - I spend 6 months outside of Thailand and 6 months inside Thailand. I timed my entry on a Non-Imm-O visa and the annual extension so the 1-year extension always expires on a January 4th every year. Thus according to this date I fly to Thailand in late September - late fall when my country gets colder - and return mid/end March when the springtime sets in. I always buy a single re-entry permit for 1000 THB on the same day my new 1-year extension gets stamped into my passport. Ideally the day of application (around Christmas) is always also the day to do the 90-days report

yes can. No problem at all but make sure to have re entry permit before leaving Thailand. If you leave thailand without this then your visa will be finish

Just to add to Tod's response, you must be back in Thailand in time to apply for your next extension, so don't leave it until the week your current extension expires (apologies if this is obvious).

No problem at all, buy a re-entry permit (either a single re-entry for 1000baht or a multiple re-entry for 3800 baht) at the immigration office where you got your extension issued OR at the airport before you fly out

you can then exit the country and when you come back you'll get stamped in until your current extension expires.