From Leagrave to London there is only one reasonable route, which is Thameslink to London St Pancras (40min). From St Pancras to Tonbridge, the most reasonable route is to stay on the Thameslink train to London Bridge (+16min) and change for Southeastern to Tonbridge (33min). There are alternative routes via Redhill (sometimes cheaper) or via Ashford International (sometimes more expensive), but both take around 30 minutes longer, so I assume you will not use them.
An
Anytime Day Return costs £54.40. If your morning train arrives St Pancras after 10.00, then you can instead use an
Off-Peak Day Return for £39.90. These tickets can be discounted by 33% if you can purchase a suitable railcard that you are eligible for. For example, the 16-25 Railcard, 26-30 Railcard and Disabled Person's Railcard apply to both tickets. The Senior Railcard (age 60+) applies to the Off-Peak Day Return, while the Network Railcard (available to everyone) applies to journeys starting after 10.00.
If you travel in the morning peak nearly 5 times a week and cannot apply a railcard discount, then a Season Ticket is your best bet. Season Ticket prices are based on a weekly price, with a monthly costing 3.84 times that and an annual costing 40 times that. Your question inspired
a thread discussing the more general issue of cross-London Season Tickets, in which the following information was posted which is relevant to you:
It's rather arbitrary which journeys have season tickets. But for journeys across London for which there is no through season ticket, there is internal staff guidance which sets out how a through season can be priced and sold.
The cost is calculated as the sum of an "outboundary" Travelcard seasons from both stations to London Zones 1-6, minus the cost of an "inboundary" Zones 1-6 Travelcard.
In the case of a LEA to TON journey the calculation for a weekly season would be £164.90 plus £154.50 less £78, so a total of £241.40. Obviously this is only slightly less than a daily though ticket, so you have to be consistently commuting 5 days a week (and perhaps going into London on the weekend) to make it worthwhile.
Since a Season Ticket allows for unlimited use in its area of validity, this custom-made Season Ticket for £241.40/week (£927.00/month) allows you unlimited travel between Leagrave, London, and Tonbridge, including unlimited use of London Underground, Overground and (almost all) National Rail services throughout London Zones 1-6, and all London Buses and Trams.
Another comment,
There's perhaps an argument here that there should be a (cheaper) through ticket routed 'not Underground'.
inspired me to look for shorter cross-London journeys which do have a "not Underground"-style restriction, and I found the following combination:
Leagrave - Knockholt (Route: Via City Thameslink) £137.00/week (£526.10/month)
Knockholt - Tonbridge (Route: Any Permitted) £73.00/week (£280.40/month)
Total: £210.00/week (£804.50/month).
Using a combination of tickets to make your journey is referred to as "split ticketing", and this combination is covered by condition 14.3 of the
National Rail Conditions of Travel: "[...] if you are using a Season Ticket [...] in conjunction with another ticket and the last station at which one Ticket is valid and the first station that the other Ticket is valid are the same, then the train does not need to Call at that station for your combination to be valid." This means that you have to pass through City Thameslink (which basically means crossing London using Thameslink rather than Underground) and Knockholt (which lies on the route of the direct train from London Bridge to Tonbridge), but the train doesn't need to stop there. I'll leave it to someone more familiar with the service patterns to comment on whether having to pass through City Thameslink and Knockholt are significant restrictions.
This saves you £31.40/week compared with the £241.40/week custom-made Leagrave - Tonbridge Season Ticket, but you do lose out on unlimited travel in London Zones 1-6. Instead, roughly speaking, you only have unlimited travel betwen Leagrave and London St Pancras, on the Thameslink route between London St Pancras and London Blackfriars, between London Blackfriars and Knockholt, and between Knockholt and Tonbridge.
If you want to use weekly Season Tickets then note that the Knockholt-Tonbridge weekly Season Ticket costs exactly the same as 5x Anytime Day Returns at £14.60, so you're better off buying those Anytime Day Returns every morning, so that you don't waste the money if you decide not to go for any reason. However, if you do this then you have to make sure you never forget to buy the ticket when you go, or else you may be charged a penalty fare or even prosecuted if caught without the ticket.
