A country mostly produced and exported low tech craps do not deserve this status.
As I've already said, this seems like a thread explicitly to bash the British.
@Thomas1195 - you are making it seem like he is right. I even contributed in good faith to this, but you could have the discussion without appearing to deride. If your concern was that Britain was producing low-technology goods and that this was a reason you said it declined, or it failed to move towards high-tech goods manufacturing, rather than "it declined and was bad" I'd say you had a leg to stand on.
An economy does not have to be producing self-sealing stem-bolts to be considered a developed economy. What it makes is only a factor. Primary, Secondary, Tertiary and Quaternary Industries are just part of the equation. Other aspects are economic institutions (i.e. Fiat Currency, Fractional-Reserve Banking, Central Banks, Paper Money).
A fair analysis of the UK at that time is that it was firmly focused around Secondary Industries (by definition it was the first country to do so in a serious way), the bulk of which produced Low-to-Middle technology goods. Excluding some of its biggest productions like ... Ships. Others have demonstrated that the British Economy WAS producing higher-end goods, but it wasn't the focus of their economy, as they were already dominating the other aspects of the market. That doesn't make them an undeveloped economy, it makes them a market leader. If my town makes nothing but silk cloth, and makes all the silk cloth in the world but still takes advantages of mortgages and payment plans, paper money etc, that isn't an undeveloped economy by any fair measure.
You may as well say that mid-western USA isn't a developed economy as they are very well known for producing crops, or Texas isn't because it is built around oil production and refinement (Primary and Secondary Industries). In contrast, the Modern UK (regardless of current politics) is the world leader in Banking (Tertiary), and is certainly more sophisticated economically than it was in the 1870s. Is it more sophisticated than China? Hard to say, but from what I've read of your position, the answer would be yes, quite a bit.
That ignores the fact that you asserted that it was declining - looking through I'm not sure I agree with your assertions now. Was their economy LESS sophisticated? No. Was it smaller? No. Not in absolute terms. Was it maintaining its dominance (i.e. its position relative to other nations), No. That however does not mean that it is declining, it means it is no longer dominant.
It is at this point that I fear inaccuracy on your part leads towards appearing derogatory. I'd recommend clarifying your position.