Hi Rudtser.
There are many points to consider before buying the airport upgrade. First, it is expensive. TG regulars can provide recent estimates, but the order of magnitude is probably around USD850 per person one way. TG is offering wonderful discount prices exLHR in C and F, especially for couples. I assume that you are using one of those. If you wished to fly F, it would have been cheaper to directly buy those discounted F tickets, rather than upgrade at airort. Plus, you might get the worst seats left or might have to sit far from each other. If you are paying yourself, a better idea might be to go back to TG or your TA and try to transform your tickets into a mixed C/F or even a F RT ticket. It will probably be cheaper.

The other point is that you wont take full advantage of BKK F product. BKKROP is right in pointing out that it is very good when you originate in BKK, but you only transit there. As you know, these are standby upgrades. So they will usually only sell them at the airports when they know which seats are empty. When there are only 8 or 10 F seats and many are already booked, they will be relunctant to sell them until they are really sure. There are so many TG feeder flights to the BKK-LHR longhaul that they will not be sure until shortly before departure. That means that it wont be easy to do it at DPS for the BKK-LHR. You probably will need to do it at the transfer desk in BKK. So you will arrive as J pax, will get no buggy to security and lounge, spend a lot of time to get the upgrade and end up primarily using the F lounge for a limited time. IF you have long transit AFTER you pruchased the upgrade, you could still use the spa, but you will just arrive from beach, so it wont be that great.
If you buy an upgrade at LHR, it would be a wonderful start of your honeymoon and easier to do at this is departure point for the longhaul and much lesser uncertainty about seats occupied. Indeed the F lounge is nothing in LHR, but you will get relaxing transit in BKK, buggy from plane, direct access to F lounge and use of spa (although no time for full massage) than F transfer to DPS lane.
F is significantly better than C because of flat seat and slightly better food/wine/service, but also because there are solo seats, so no disturbance from a neighbour. But as a couple, C seats are nice as you sit together and have no neiboughs. So think twice whether it is worth the extra $1700 for a one-way.