Originally MGs were named with the suffix 'Type' (like E-Type Jaguar), and occasionally you would also get a word to denote the cars size. Thus you had the Y-Type, T-Type Midget, P-Type, N-Type, Z-Type Magnette etc.
When cars were facelifted there was an additional letter added, thus:
TA, TB, TC, TD, TF Midget and ZA, ZB Magnette, YA, YB Saloons etc.
However, with the introduction of the Y-Type saloons and then these were followed by the Z-Type Magnettes, in the 1950's MG had run through the alphabet, so, with the launch of thier new sportscar they decided to start again, but change the format a little.
MGA was the first of these cars and was followed by the MGB, MGB GT (GT version of the MGB), the MGC and MGC GT
In the 1970's it was felt that BL didn't need 2 competing sports cars in thier portfolio. The MGD was designed and pitched against the proposals for what would become the Triumph TR7. Sadly the TR got the green light and the MGD never got off the drawing board.
MG would have died if it were not for the badge engineered Metro, Maestro and Montegos of the 1980s, say what you like about them, but remember that if it wasn't for them there would very likely never have been another MG (look at the last Triumph, the Acclaim - where is the Triumph marque now?)
In the mid-80's there was a supercar concept called MG EX-E.
In the late 80's the MG RV8 was designed - this was a 'toe in the water' exercise to guage if there was still interest enough to bring back an MG sports car. It was an MGB, updated for today with a Rover V8 engine (R V8).
So we come to the MGF (MGA, MGB, MGC, MGD, MG EX-E), F was the next letter in the sequence.
The TF, is an unusual case - it makes no sense at all from a traditional model naming point of view, other than MGG doesn't read properly (ho ho, one horse power eh? GG geddit?) and with the introduction of the Zed cars (ZR, ZS, ZT) MGR had reverted to the earlier 2 letter naming strategy.
So, TF doesn't stand for anything - you could argue that they wanted to retain the F to link the new car with the MGF, but where they got the T from is anyones' guess. It is doubly confusing as there was already an MG TF in the companies history (maybe they did it on purpose?)