The Essex co-stars were among the guests invited to afternoon tea to celebrate 150 years of the Barnardo's charity, which they've both worked with on campaigns.
Lydia, 26, shared a beautiful picture of herself and Debbie standing outside the palace gates on Thursday morning (12 May), commenting: "What an honour."
She's just back in the UK after a sun-soaked holiday to Bali, and was looking very bronzed in her pretty blue and white embroidered dress which, naturally, was from her own Bella Sorella store (available here for £89).
Lydia and Debbie have both worked with Barnardo's to raise awareness of fostering. Debbie has fostered more than 250 children over 22 years.
On the Barnardo's official website, Debbie says: "I was first inspired to foster in 1993, by a chance meeting with a foster mum in Pizza Hut. We swapped phone numbers and eight months later I fostered my first baby – a gorgeous baby girl, aged three. Lydia and her sister Georgia were three and four at the time, and they adored their new 'sister'. Since then, I've fostered all ages.
"I have four other children, and at times I worried about whether I was doing the right thing by fostering," she adds.
"But my children tell me they love their big, chaotic family and wouldn't change a thing. Lydia says it was like being on a constant school trip, bunking up, sharing stuff, just being part of a huge, fun family."
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