I am sure it's the right Mabel - just not certain it's the right Reginald in view of the birthdate and commonality of the surname. My advice was to buy a copy of their marriage cert to check his fathers name & occupation and it may also show Reginalds age when he married.
Freebmd have 2 births in the June qtr 1900 and one in the Sept qtr - none in Berks. Mabel was b Somerset so who knows 
Reginald’s, birth 1898
BROWN Reginald qtr 3, vol 2c, page 250 mmn RICHARDSON.
Kingsclere is the registration district for East Woodhay where he was living.
Reginald’s parents marriage.
The Parish Church Burghclere 27 Nov, 1890 after Banns.
Tom Lidston BROWN 25 bac, Carpenter father Thomas BROWN labourer.
Louisa RICHARDSON 22 sp, father William RICHARDSON labourer
Witnesses William RICHARDSON & Ellen RICHARDSON.