Although the OP says he doesn't want a SIM that requires monthly top-up
the advantage of GG is that you run your account on line all by yourself.
You need to establish a £10 credit at the start but you can set a low
limit for the account to top itself up, typically £3, and by whatever
amount you instruct.
If going on holiday - UK or abroad - then buy a 'Goodybag' which (apart
from the cheapest) will give you unlimited calls and texts and whatever
amount of data you have selected. This can be set to auto-refill every
month. They will send you a text before it tops up, and an e-mail to
review your usage that month usually two days before a new Goodybag is
purchased to give you time to change the amount or switch the
auto-refill off. They then send you a text when the new Goodybag is active.
The good bit is that, as I said above, you can change any of these
selections at any time just by going into your account from anywhere.
Also remember that when abroad (well, France anyway) you can usually get
on wi-fi free of charge at the local tourist information office or
outside it if it is closed. In many small places there is wi-fi
available in the vacinity of the Marie's (mayor's) office BUT, unlike
the TI access which is not restricted, the facility is usually provided
by Orange France (i.e. France Telecom) and they block anything and
everything except browsing. So if you use webmail no problem, but if you
use a mail client such as Thunderbird you will be blocked.