Wilson, Wallington, Sutton Grammar
Theoretically, yes, there is nothing to stop OP from applying but it is over 20 miles from Harrow to Sutton and according to Google maps roughly a 1.5 hours journey each way, and that's outside of rush hour... Plus a certain proportion of places in SGS and Wilson's are offered to boys in local 'preference' postcode areas. Not sure whether Wallington do the same at present. Best to check the latest admissions policy documents on the respective websites.
Registration for both boys' and girls' Sutton grammars' SET in September is now open, so parents of children currently in year 5 can start applying now. Note the deadline for the SET registration is earlier this year, on 19 August. The first round test (SET) date is 14 September (according to SGS website).