@Jim Newell,
I appreciate your worry over your discovery of your US tax obligation. Not that it helps, but there are many US citizens around the world going through stress similar to yours at this time. You're getting some bad information from various sources.
Let's start with the exclusion for personal residence. The amount is not a flat $500,000. The exclusion, if you qualify, is $250,000 per each US taxpayer on your return. If you were to file married filing separate (as most US cits. with an NRA spouse do), there is only 1 US taxpayer. If you were filing married filing jointly (Your spouse would file with you, and the return would include the incomes for both for US tax purposes) then the allowance is 2 X $250,000, or $500,000.
Renunciation is not dependent on filing a tax return. Your nationality/immigration citizenship is separate to your tax obligation via US citizenship. You may renounce US citizenship (your tax status has no bearing on this and will not be questioned), but without filing the last 5 tax years and form 8854, you become a covered expatriate. If you are a covered expatriate, then the tax obligation of those 5 years (plus penalties) follows you for the rest of your life, and in some cases, after your death. Research, research, research.
£30,000 is a viable figure IF you have a considerable number of UK accounts which can be classified as PFICs. Others have paid such a fee, but they had many, many PFICs. The cost of simply preparing 5 years of past tax returns, form 8854, and a final 1040/1040NR should be no where near that amount if your finances/pension arrangements are straight forward and simple.