The growth in the Uckfield line was mostly in the 3 years after the new trains and direct services to London Bridge were instituted.
Since 2007/8 it has been a rather more modest 3.7% on average, which I guess is broadly similar to the LSE average across the sector. Growth in the last year that numbers are reported (to 13/14) was 0.57%.
I think to a certain extent the current services have reached a decent saturation level. From speaking to the passengers on routes via Oxted almost every working day as I do, the current service level is used by almost everyone who wants it. It's quite rare that you find passengers who are completely unfamiliar with the route but they may just use it more regularly, including versus Tunbridge Wells or Haywards Heath/Lewes routes.
There are certain things which passengers tell me would lead to "maximum appeal" of the current route, and probably completely fill it to the brim. This may need to be done before it becomes a no-brainer to have the massive upgrades on the existing route that any extensions South of Uckfield/North of Selsdon Road would need. These are the reduction of services operated as 2 coaches (some passengers don't even want to board these at Uckfield because of how warm and solidly wedged they can get - but of course if 170 refurbs go as planned, this should be minimised) and later evening trains (not
necessarily viable without rewriting track and stock maintenance options, but AFAIK something that is looked at from time to time behind the scenes). Better service in the peak to Hever and Ashurst occasionally feature (early morning services etc.). Ticket prices do attract a lot of people, and most passengers seem to like the rolling stock. It may have had some rather significant reliability issues of late, but mostly this has not affected the baseline of how comfortable it is. And rail passengers in the area seriously despise buses, for the most part.
Things which would help get accurate passenger numbers recorded would be a decent level of revenue support between Edenbridge Town and Uckfield, in order to map demand. This is persistently a problem and something which is drummed into new train crew, but can't be rectified without a manpower increase. A possible issue with late-night services is that plenty of people board South of Hurst Green but they are very rarely actually sold tickets. Bear in mind the timings of ticket sales are very closely monitored by GTR.
All of these local issues would do well with being resolved before or at the same time as any grand BML2 project. In fact, solutions would probably be very valuable as continued groundwork for the viability of infrastructure proposals such as double-tracking, extension to Lewes, later evening services, and so on. It would prove desires amongst both GTR and passengers to continue the growth of the line. There are always constraints, but maximum appeal of the current service has not quite yet been reached, and until then, huge additional improvements to me come second.