Here is an alternative option that may be worth a look:
Slough - London Paddington - London Bridge - Redhill
--- split ---
Redhill - Tonbridge - Ashford/Hastings - Rye
This saves almost £15 on the HS1 fare, and £8 on the non-HS1 fare. However, NRE is quoting a journey time of roughly 4½ hours, even though direct services operate on this route between London Bridge and Tonbridge (admittedly only once an hour though)...
In other words, stick with the £63 non-HS1 option. At least that way you can get a direct service through to Ashford from Charing Cross and change there.