Interesting, indeed it does seem we have quite a bit in common re: work. I also work from home but drive down to the New Forest to the office once a week. I don't do much UK customer visits but do assist our sales guys when needed.
The wife also drives a mini, a cabrio S though, and we use that for any inner city jaunts locally or when we want to go down to Devon and the beach for the two days a year the sun does shine, the winds are light and it is warm!
She doesn't drive the 7, in fact she hates it but is happy enough to be chauffeured around by me when needed
I have the stock 18" wheels on the 7 right now and as they aren't flush with the tyre, it is quite good for going around tight spaces or low walls/high curbs in the UK as you say...I will eventually put some Winter tyres on this set of rims and am thinking about buying a set of 20 or 22" Vorsteiner rims with Summer tyres but these would stick out beyond the tyre and I'd have to be very careful indeed here in the UK to not curb them...
For your circumstances, it seems you could easily get the 7 and enjoy it and as the kids are older, they would understand that Santa doesn't come to naughty kids as you say...
One thing I did to help me decide, was to rent one (albeit on the continent) for a weekend and drive it around to see how I got on and if I really wanted the comfort and luxury over a sportier car...You know the outcome of that! However, it does help to focus the mind and you can rent a basic 7 for roughly £500 for a weekend (I went to Portugal and the Algarve to do my test runs)...
Do let us know what you decide, lovely cars either way really. If you're ever down Bristol way, let me know, happy to meet up for a coffee/drink and car talk!