Which credit card is best?

The credit limit will be decided based on numerous factors such as income, current amount of debt, information from credit reference agencies and internal lending criteria.

Whilst it is true that some lenders are more "generous" with credit limits than others (and I use that word with caution), the above factors that I mentioned have the greater influence on which band of credit will be offered to you, on a more general scale. For example, someone with no, limited or adverse CRA data may face limits below £500 across subprime whereas others with an established & credible record could be offered limits of £1500 and much higher on mainstream.

I cannot speak for Halifax, but Nationwide present a credit limit up-front before making a full application, although it can change or decline once final checks are conducted, ranging from credit reference agencies to fraud prevention databases, however the limit will usually be upheld if everything matches up. This will at-least provide an idea of the kind of credit limit you would be offered.

From the estimated expense of a trip to America, of such particular state and assuming this holiday is being entirely financed on a credit card, I would hope for a limit of £5000 or higher for a comfortable trip, unless spending money is being financed from your own pocket.

On an unrelated level, but I couldn't resist; if I was a married woman and my husband took a "lass" to America whilst I was on a training course, I would not be best pleased, especially in the context it seems from your choice of words.