Hi,
As far as I know if you have 1 kWp installed, an inverter like the
fronius IG 35 plus will be fine for up to 3.7kWp at a later date which is a fair amount!
I have a fronius IG 20, it kicks in at less than 40 Watts produced by the panels. My 2.1 kWp are in 2 strings so I was told that I I want to add to that I'll have to put another string of the same size (5 panels).
As for the efficiency resulting from under-sizing, you could make a simple modelisation using
PVSYST. All the inverters up to 2008 are in the data base of the program along with the panels and weather situations.
About the size of the install, I initially wanted to do like you, but make sure that you get the maximum grant as possible (£ 2.5 k + whatever your local authority offers) as you can only apply once.
Good luck with your installation, remember to put into the equation the pleasure you get every sunny days (and even not so sunny ones) when your panels are producing and that your house is self-sufficient, solar coffee etc...
Welcome to this forum also,
Stef
