Fairly good indication of what the middle & upper classes are naming their babies: announcements.telegraph.co.uk/births
I'd expect Weston & Rowan to have siblings with names like...
Girls' names: Sorrel, Bronwen, Catrin, Cleo, Erin, Merryn, Manon, Jessamine, Tamsin, Megan, Coral, Darcy, Adelaide, Imogen, Evelyn, Scarlett, Pearl, Olive, Harper, Juno, Ruby, Sadie, Jasmine, Flora, Bronte, Saffron
Boys' names: Fraser, Fergus, Angus, Cormac, Aaron, Austin, Declan, Ewan/Euan, Flynn, Tristan, Holden, Reed, Heath, Griffith, Aubrey, Digby, Quentin, Willoughby, Lachlan, Blake, Ellis, Miles, Beckett, Bennett, Kieran, Damon, Emmett
Some of the above are much more unusual than others. The names have a fairly unisex, nature, surname vibe.
For what it's worth, I'd spell Sydney 'Sidney' for a boy and 'Sydney' for a girl.
I like your current choices although, with the exception of Savannah, I think they are radically different in style from your twins' names. If you like them, great but don't feel you have to drastically alter your own tastes to suit life in England. Amelia and Oliver are massively popular so I'd avoid those ones after two unusual picks.
Stella isn't used much in the UK due to the alcoholic beverage and its unfortunate associations... Matilda is fab and on the rise. Savannah is a bit American but if it's good enough for the royal family then what can I say? I quite like it myself. Susannah is a great alternative if you want something more classic. Susie is a cute nickname.