I lately did a lounge crawl at LHR.
OS operated flight in Y, TK Gold card.
No issues to visit all the 5 lounges.
Spent time in the late afternoon/evening, not sure about the breakfast spread.
My personal opinion; winner is SQ with best food (hot and cold), most comfy seating and great beverage selection.
UA had similar food offerings and a wonderful bar service.
If you like marshmallows and beer on tab go to Maple Leaf.
LH - oh well not sure if they have gummy bears in the morning I’m sure though the Senator section will have some hot items.
If you have enough time, definitely check out the satellite.
If timing is tight, why not have a full English at the hotel or at the airport?
Sorry can’t comment food on board.
It was however very disappointing to find out OS only serves a bag of crackers in Y on the 2hr evening flight whereas with BA you would have got something like a wrap.
And no hard liquor in OS either! Few pics below:

LH Senator afternoon nibbles
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SQ drinks menu
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Famous Ice Cream
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UA Drinks


UA Bar
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AC - beer anyone