The best mexican/texmex chain in UK was without a doubt Texas Lone Star. Every branch had the same legendary menu and quality and every item was top notch. Then the owner passed away and with him, within weeks, the entire chain. You can still find 100% five star reviews in some of the older rating sites.
The next best tex mex was Nachos chain. Bigger and better with every branch at some point in the late nineties the chain was purchased by Garfunkels (the syndicate behind various stake houses around the country), within months of introducing their new budgetary and economic policies majority of the Nachos branches lost their customer base and shortly later the whole chain closed down.
The third on my list was the most expensive of the lot, but generous in size and menu - Chilli's. American owned chain, superbly organised and with well prepared food. As the US tumbled into economical disrepair in 2008, the British holdings went into administration and UK operations closed down before decade was over.
Since then all of the tex-mex fusion and para-mexican places as found in capital and south east were nothing but disappointment to my taste buds. Yes, there is the touristy Chiquito's chain, which after several menu adjustments now serve what can only be described as glorified but minimalistic carbon copy of TGI Friday's offering at TV chef restaurant prices. There is the eclectic Chimichangas which degrades to microwaving their freezer derived starters (at least our local branch in Kent does that and is lazy enough to melt cheese under halogen lamps as they wait for main away). And the various take aways and "street food" wannabies, but in my book if food is not served with pitcher of frozen margarita or six line of 100% agave tequila shots, it can't even count as mexican restaurant (shrugs).