A typical UK-based tax accountant who knows about both US and UK tax is likely to charge upwards of £250 per hour for their own time, and £150 per hour for junior staff time, all + VAT. I expect no accountant will do a US tax return for less than about £500, probably more. So the price of $350, quoted by US-based people is quite competitive, and there is no VAT to pay. However, it sounds like your affairs might be very simple. Why not pay the $350 once, in a year that you have the complication of a dividend, see how the professional accountant does it and then in future years do it yourself, substituting your new numbers in the right places? You could use software like TaxAct ($29.99) to help with filling in the forms.
Something to consider about dividends and interest is that from 6/4/16 there will be no UK tax charged on the first £5,000 of dividends or first £1,000 of bank interest (for basic rate taxpayers.) But such income may incur a US tax charge, unless you have paid UK tax on other income which can be credited in the same Form 1116 baskets. This mismatch between tax systems will add to the costs of being a US person living in the UK, additionally to the $350 cost of having a tax return prepared.