As you imply knowledge of in your question, doing what you propose is a long-term currency bet on the strength of the USD relative to GBP. As such, any question of profitability should be dominated by the relative exchange ratio of GBP/USD over your holding period of USD-denominated assets if your doing this right. For example, assuming your considering either a large enough capital position or a large enough rate change, both combined with relatively low fees.
As to exchange costs and fees, I agree with @keshlam's comment to the question. But I'll add that these costs also vary depending on how you go about doing the exchange too. Can you just buy US bonds in your current UK broker? What are their policies on currency conversion to do so? Or are you thinking of actually separating the currency conversion from the trading of bonds, perhaps by opening a US brokerage account separately? If the latter, you should be able to use one of the many low-cost, independent (non-bank), online, currency exchangers. These generally have lower fees than banks -- which could save you thousands of £/$! Another alternative is to see if your broker will allow you to trade FX at reasonable costs. Mine does, I pay $2 commission for a GBP/USD trade at market spot rate with a <1 pip spread, and only have very minor restrictions on deposit and withdrawal abilities. Much much better than a bank! And much cheaper than my broker's policies if they do the conversion themselves too.
As to taxation, the US and UK have a taxation agreement so that you won't be double-taxed on any profits from your investments (coupon payments on your bonds during your holding period, capital gains when it comes to sales of your bonds.) And the US rate is actually lower than the UK rate, though you may not have a choice in which you have to pay. Anyway, my point is that you shouldn't incur "high US tax liabilities" as compared to your existing GBP holdings.