A one-way way is ludicrously expensive, as it only prices up in full J at £1200.

However IIUC, you are trying to book an open jaw LHR-LED in CE // DME-LHR in ET.

Provided that R class is available on the outward and you are meeting the R class rules (eg Sat night stopover), you should be able to combine this with a cheapie O class from DME-LHR for around £400.