I'm sure there used to be BA flights to GDN and it still comes up on the drop-down but I can't find it (they used to fly to POZ at one stage for that matter as well but I think that's long gone).

I suppose you could fly LHR-WAW on BA and then catch a domestic LO flight to GND and catch a cab from there as an alternative.

I generally do not mind the Polish trains though - they tend to be pretty reliable and some faster trains are rather comfortable, so I'd probably be tempted to fly LHR-WAW on BA and catch the train, personally, if I wanted to stick to OW.

There are of course a number of direct flights from the UK to GDN if you do not mind flying with an LCC. It's an option well worth considering for potential time-saving depending on where your nearest airport is etc.