The issue with buying BTC has a lot more to do with the specific exchange than the theories around how currency pairs fluctuate in value.
If there's no exchange in the UK trading BTC against GBP directly, then pick a different fiat currency. Whether or not that may theoretically yield you less BTC than a direct GBP transaction is irrelevant if there is no exchange trading against GBP.
Various exchanges have WIDE ranging order books between BTC and fiat currencies, USD or EUR or RUR. And the exchanges might not even have the liquidity in the orderbook to fill the amount you're trying to buy anyway; anywhere near the listed last price. One exchange I use frequently has +/- 30% listed for BTC/EUR simply because the bid/ask spread is so wide (because there's no real liquidity in that pair on that exchange).
In fact, here are the price quotes right now for BTC/USD and BTC/EUR on the same exchange.
Pair Price Volume Change
BTC/EUR 2500.0400 0.1800 -26.47%
BTC/USD 4178.00000000 1520.1300 +4.61%
Right now X-Rates lists one EUR is trading for $1.180922 USD. So the quotes above bear no relationship to the broader forex market, and reflect a clear lack of liquidity on the EUR pair on that exchange.
If I were you, I would find someone on a local forum to buy coins from with cash. Separately, and I'm sure this is obvious but it's worth reiterating, if you're having this much trouble buying while BTC is hot, consider how hard it will be to sell if it's ever not. (This is coming from someone who has been involved in crypto and is long BTC, make sure your risk level matches the reality of the state of liquidity where you are)