The reasons are multiple, and have to do with both the direct effects that the war had on U.K. and the choices made politically after the war.
Firstly, at the end of the war Britain was a "Victor" yet saddled with the tremendous debts that fighting the war had cost. Some of the mechanisms that had kept it afloat and able to carry on fighting such as the lend-lease act was cancelled on 21 August 1945 . At that time U.K. still had it's army spread widely over it's still existing empire, yet no longer the means to fund such a vast undertaking. It's economy was at this stage geared fully for war production, meaning that exports were nowhere near suffucient to cover imports needed. The cost of maintaining it's military obligations, the needs of the population as well as the debts accrued during the war put it in a bad position from the outset.
Politically the ruling parties post-war took too long to scale down their ambitions regarding U.K.'s armed forces, and used bridging mechanisms. The Anglo-American loan of 3.75 billion USD was negotiated in 1946 (and finally paid off in 2006). The money from the loan was for the most degree used to keep supporting the U.K. military's foreign undertakings, as well as some vital imports. Due to the inability/unwillingess to scale down the army and it's foreign engagements the money was spent there, rather than for reconstruction at home.
Once Marshall aid started flowing (U.K. got more than Western Germany, despite the latter being mildly put in a worse of state after the war) there was yet again political decisions taken that did not serve the population of the country well. A large amount of marshall aid was used to prop up the Sterling Area mechanism. Food and tobacco imports also caused a drain, the trade balance still being negative and yet again some marshall aid was diverted to military endevaours albeit indirectly (fuel supplies for example). Compared to West Germany there was at least 30% less investment in industry out of the Marshall plan funding.
Sources: The Lost Victory: British Dreams, British Realities, 1945-1950 by Correlli Barnett
The Path to European Union: From the Marshall Plan to the Common Market [Hans A. Schmitt]
[edited for mistyping of numbers]