That seems right. These were very common surplus after WW2 and even, to a degree, before. They were cheap back then because .455 Webley was hard to find here. But surplus .45acp was common and cheap. So the conversion was very popular here in the late 40s and into the 50s. Most of them had been sold off by the 60s. But as a result, unmolested and original .455 caliber models are worth a whole lot more, as not many survive here.
If yours has some sort of documented provenance as to where it served and with whom and when, it would be worth more, even in converted condition. But that seems unlikely. Because these were cheap back in the day, they ended up in a lot of criminal circles, just like inexpensive guns today do. But just as equally could have been a shop keeper's companion in some Mom & Pop store.
Without inspecting it beyond the picture you include here, my guestimate is that it is worth about $600-$700