SHAREs - Buying in on the NASDAQ ?

On the W-8BEN:

5 you can leave blank if you don't have one;
6 you can use national insurance number or UTR;
7 leave blank because if you deliberately give them a specific reference in the form they should only associate it with that specific purpose (and you might for example want to have an ISA account and a separate general investment account). If you are sending it to them by pose or email you will presumably tell them who you are and why you're sending it to them.

10 is only to be used if you personally meet a specific criteria in your country's tax treaty which entitles you to a special lower rate of tax withholding than a normal person in your country of residence would get. So for example if you were a UK parent company which owned more than a tenth of the voting power of the US company from which you were getting dividends, you could get access to a special low 5% rate of withholding on those dividends instead of the normal 15% that the rest of us need to suffer. You're not, so you don't

You'll find all the UK brokers that are dealing in international markets generally charge more than X-O.co.uk would charge for UK trades, because X-O are deliberately offering a no frills UK stock exchange only service, which is cheaper to provide than an international dealing platform.

As mentioned up the thread, ii have access to lots of overseas markets and a convenient multi currency account functionality which a lot of other UK brokers don't bother to offer, although FX commissions are still reasonably high. The ISA rules prohibit them from allowing multi currency cash in an ISA though, so it's only an advantage for non ISA holdings . Their fixed ongoing fees for having the account (see thread on their recently-announced fee increases) is higher than it used to be which may put you off if you're a long term buy and hold sort of person, but effectively it's an inactivity fee because the monthly/quarterly fixed payments you make are available as trading fee credit for a limited period.

If you're not too hung up on needing multi currency facilities there are a number of other brokers or DIY investment platforms that can handle NASDAQ / NYSE stocks. I have international shares in both my ISA and SIPP with AJ Bell Youinvest and they are fine, though sometimes a live online quote isn't available which can be annoying as there's no facility to leave an online 'at best' or 'limit' order on non-sterling stocks; and telephone orders cost £30 a time.

Another service is IG.com, they have decent systems though I've only used them for spread betting and CFDs rather than actual shares.

If you don't want ISA or pension facilities and are just dealing in a general account with no protection from the UK tax man, you won't need to restrict yourself to UK brokers and you could try other European service providers such as Saxobank or Fineco. Some like Degiro are super cheap, though if investing large amounts I would generally look at trusted UK names so you are dealing with UK institutions if something goes wrong