The 70-300 gets some moans from UK based people that it struggles to focus in low light - it hunts (slowly as it is not that fast at focusing anyway). Others claim to have no such problems, as is typical in gear arguments

There is a UK based hire company for four-thirds kit (details
here). You could hire one for a few days for relatively little outlay and see for yourself if it suits your requirements.
The 70-300 is selling for around $200 in the states at the moment yet it's still £300'ish here :annoyed:
If I had been able to find a
second hand one for under £200 recently I could have let you know how it performs myself but £300 when I already have the 12-60SWD and 50-200SWD is not going to get past the wife (especially as currently unemployed).
Another possible choice is the 18-180. That gives you wider and longer than your current lens. That option would probably be a replacement for your 14-42 rather than an addition.
If it is an improvement in lens quality you are looking for rather than adding options then you have a choice between the 14-54 and 12-60. The difference in quality to the kit lens is quite marked but so is the cost. The difference in quality between the 14-54 and 12-60 is not as big as the difference in price in my opinion although the spec sheets and some "experts" disagree with me

While I have the 12-60 I would not be upset to have the 14-54. My 12-60 doesn't get used at 12mm much anyway due to the completely wacky distortion being a pain to process out afterwards.