Asset alloc: if you have any UK domiciled stocks, they are most efficient (least bad) to hold outside the ISA as there is a dividend credit. Most ETFs are not UK domiciled, even when they hold UK stocks, sadly.
Bonds are usually worst outside a tax shelter.
http://monevator.com/compare-uk-cheapest-online-brokers/
I'm searching for platform to open my ISA and start investing into index funds. I stumbled onto this thread and started to wonder, why you are discussing so much about funds being domiciled or not? Does it really matter if I'm going to invest only 15k £ per year through ISA and none outside of it?
Ok, so here's how it works. There are foreign taxes. If foreign taxes are paid, assuming the tax is paid outside a tax shelter, you can offset your local tax - assuming there is a double taxation agreement (there usually is). So, for example, if you hold US stocks, dividends will be cut by 30% - or 15% if you have a W-8BEN on file.
In an ISA you can't get that money back any how. In a pension, there is an agreement where nothing gets withheld.
To the best of my knowledge - in an ISA - it's gone. Lost to the US government. If you have a LSE listed ETF trading in pounds that holds US stocks it is probably buried and gone. If it's listed in the US, and bought in US$, outside an ISA you should be able to reclaim it.
All this is moot if you're only investing in an ISA. Don't let it worry you. Honestly, I'm not 100% on the various tax arrangements within an ISA, I'm not sure how it works with things you buy in pounds that hold foreign stocks. I know that Irish and Luxembourg domicile impose no extra tax. Obviously you want to avoid double taxation so probably holding something in US$ is going to have less in terms of foreign withholding - with a W-8BEN - but the cost of currency conversion makes it more a pain.
I'm much more 'into' Canadian stuff, as that's where I live now. The number one thing is to start investing, doing what your asset allocation says, reinvesting any distributions.